The downside to the A9L is it can be hard to tune because of the complexity of the strategy. Megasquirt is much easier to tune because it doesn't have all of the emissions or complicated edge case drivability dependencies. Holley is easier than Megasquirt.
Cougar 5.0 went Holley, but his exact route requires putting in a carb intake and a full re-wire. (Correct me if I'm wrong!) Holley has other options, but I'm not super familiar.
Personally, if you aren't interesting into dumping a bunch of labor/$ into your car, I'd try and find a shop (even if it's down the road a bit) and tune your current A9L as you're N/A and don't have an extreme setup.
I used a carb-style Edelbrock SuperVic with the injector bungs because there are few choices in feeding a 427w to 600HP that will also fit under a stock hood.
I initially tuned the 427W using the A9L for break in and until I had all the Holley stuff gathered (I bought it used and a piece at a time cuz I'm a cheap . I even made the main power and a couple other cables myself. I did save $1000 tho :) ) With the A9L, I couldn't tune out some bad surging below 2000 RPM caused by the larger cam I'm using. I tried a few tricks that helped a little, but I just couldn't solve it. Been tuning for over a decade with the TwEECer, so it wasn't for lack of experience; as noted it was just too complicated.
The Holley does replace the engine harness, which after 30+ years was kind of nice actually. The new wiring is nice looking and no more Salt & Pepper shakers to deal with. Also, since the Holley logs many parameters by default, I could remove a ton of patch wiring for sensor logging including an old Innovate LM-1 wideband and DataQ acquisition module, USB hub etc. A mess feeding my glove box that I'll never have to worry about again. Fortunately, Ford has separate sensors/cabling for the dash gauges, so nothing lost other than the trip computer which used a feed from the Ford ECU.
Because the Holley uses full time wideband feedback, not only will it learn and get the fuel dialed in (using one of their base tunes as a starter), but with a bigger cam diluting the mixture at low RPMs, a secret I learned is to enrich the mixture there to ride out the lower RPM surging. Difficult to impossible to do with A9L, so it's been a worthwhile effort for me.
Holley this year released a Terminator X unit which is much less expensive than the HP/Dominator units.
Now, for a mild setup as per this thread, a Moates Quarterhorse or TwEECer can be used, or just a properly calibrated MAF as noted above.
I'm not familiar with Megasquirt. I chose Holley EFI over continuing to tune using TwEECer and A9L. The Holley works well and I found it easier to tune around the larger displacement and larger cam compared to A9L.
So, I made a little video of the modified tach in action. I even free-revved it to >5k RPM so y'all could see. Seeing as it reads accurate now, I had to really stab it hard to get it to kiss 5k RPM.
I'm guessing some know how to remove the tach module from the dash, but for those that don't:
Remove the clear plastic face from the dash (6 screws.) Flip the dash over and remove the 3 screws which hold the tach module to the dash, one is in the middle center (behind the tach) and two are at the top on either side of the illumination bubs for the tach. Next remove the screw holding the ribbon cable to the dash, then carefully separate the ribbon cable from the dash which is held in place by two white friction-fit posts. Now flip the dash again. Remove the Lamp Out and Check Engine light sockets (twist type) from the lower left face of the black bezel, then pull the ribbon cable off the two black friction posts. Lift the top of the bezel off the 3 white posts and note that there's a rubber pad at the bottom that fits over the steering column. This pad ties the top black bezel to the white dash body and can be rather annoying to separate from the bezel as just when you think it's going to go, you note there's a tongue on the bezel that interlocks with a slot in the dash body and this dumb rubber pad is stuck to that tongue. I probably should have just used a razor to make clean cuts to separate the two, but one way or the other you need to un-stick this very sticky rubber pad that likes to tear apart under stress. Once you can fully lift the bezel, you'll note that there are two multi-wire cables that connect to either side of the dash from the bezel. You'll need to disconnect the 5 pin one (only 4 wires) that connects to the tach (left side.) It's taped so you'll need to cut or remove the tape first. Once that cable is unplugged, fold the bezel back and grab the tach module and lift it. Note the ribbon cable will require you to 'slide right' to remove the tach module cleanly. Fold the bezel back onto the dash and put it somewhere safe.
Disassembling tach module and replacing crystal:
Firstly the back part of the plastic clam shell must be removed from the tach module. This part stressed me the **** out and I'm assuming it'll be the same for others. There are two tabs that hold the (probably yellowing) back of the tach to the rest of it. Both tabs must be pushed in to then rotate the back piece out of the way. I used a screwdriver to push one side in (easy), then try not to pull it out to far while working on the other side. It takes patience and you may wonder if you're going to break the plastic tab off, but it'll even eventually pop off. Once the back is off you'll need to carefully slide the blue connectors on the ends of the kapton (brown/orange) ribbon cables from the right angle connectors. There's two per side. Try to slide them off evenly so the pins don't get bent. Once all 4 are removed, use your thumb and forefinger to hold the ribbon cables out of the way while you lift and remove the stacked printed circuit board assembly from the tach. Turn it over and you see this:
The final step before desoldering and replacing the crystal (oval shaped shiny metal can) is to separate the two printed circuit boards (PCBs) that are held together with little plastic stand-offs. Carefully wedge something between the PCBs where no components are and pop the boards apart. Start at the end away from the little white ribbon cable that connects the two boards together. The boards can then be "unfolded" being careful not to damage the ribbon cable connecting them. Now you have access to both sides of the PCB so you can desolder and remove the old crystal, then solder the new one back in.
Reassembly is much easier, just be careful when finagling the ribbon cables back into place and don't forget to plug the 5-pin cable back together also. Might be a good time to clean up the sockets and replace some of the finicky dash bulbs. I recommend getting a can of CRC QD Electronic Cleaner and using it on all bare copper/tinned connections.
Oh, and try doing a post like this on short attention span Facebook
I did buy 10 crystals just in case I'm tasked with helping out someone who isn't comfortable doing this (for a fee, it's a lot of work to do as a favor.)
Yay. It worked like a charm, see the attached pic. You'll have to zoom in but it's tracking perfectly with the Holley EFI dash. I free-revved it to the yellow and it was nice & smooth. Can't drive the car tonight because it's wet out there and I'd kill myself, but I was more than pleased to know my tach is calibrated now.
Well, I finally got home Monday night and my first chance to try to new oscillator was last night.
It worked as expected. The only issue I saw is now that it's sampling up to a higher frequency (5k RPM is now 333Hz vs 250Hz), it had some issues at higher RPM when the (positive) duty cycle was less than 20%. I couldn't really measure the actual duty cycle at the coil (engine running) with the cheap portable oscilloscopes I have, I'll just have to try it with the dash hooked up to the signal from the neg of the coil and see if it lights smoothly to redline. Will update assuming the weather cooperates enough to let me try this out in the next few days.
I'm using the 2003 Mach 1 springs front and back. I ended up putting a 1/4" shim on the front due to tire rubbing on road dips.
Was it rubbing due to tire/wheel selection or just because it was too low? When you say a 1/4" shim, do you mean wheel spacer or under the spring?
I would like to see a pic of the stance you have.
Yeah, 17x8 wheels with a 235(or 245)/45/15 (I think, whatever ratio get it close to 25-26" tall) were a little wide so it was kissing the inner part of the fender lip sometimes on dips. Yeah, aluminum spacer under the top of the spring - see picture below.
This is about what the stance is currently:
It may be a little lower in the back to accommodate launching at the dragstrip.
Hmmm, I may have a solution if the microprocessor will cooperate. The silver can in the above photo is a crystal oscillator. Sets up the timing for everything, including the sampling of the pulses from the negative of the coil (or whatever tach input you're using.) It's marked 4 MHz. I think it counts the number of clock cycles between spark plug fires, the less pulses counted (higher RPM as plugs are firing more frequently) the more bars you get on the display. I needed to think of it the other way, the more clock cycles counted between spark events, the less bars you'll see. So if we have more clock cycles for the same engine speed, it should mean a higher clock speed should show less bars at the same RPM, which is what we want. So, my guess was that if we increase the frequency of the clock to 5.333 MHz (it reads 250RPM high @750RPM = 250/750 or 33% error), it would then read accurate. I wanted to verify this before ordering a crystal. I have some crystals from the "crystal upgrade" thing that I was going to do to my A9L, so I tried sticking one in there. Went crazy, 18MHz must be beyond what it can sync to. I then realized that my function generator could run to above 2 MHz. I maxed it out at 2.16 MHz and then tapped it into the crystal location. I then realized I had no way to simulate the pulses from the ignition coil Wait! I have this old tube audio frequency generator from like the 1960's! Now I'm definitely looking like a mad scientist.
I was hoping the input for the tach was robust enough to handle a 50 Hz AC sine wave. I tested it with the original 4 MHz crystal and it worked. Sweet. I then removed the OEM crystal and injected my 2.16 MHz signal from my function generator. It powered up fine, and then when I connected the 50 Hz signal, I saw 9 bars on the tach! I sat down at my laptop and did the math - it should have read 9.2 bars, perfect. So, assuming the micro can sync up to the higher 5.355 MHz (closest frequency available) crystal that I'm ordering, it should read pretty much on the money.
The last experiment I did was to confirm that it reads continuously up to 5000 RPM (3rd yellow bar) with anything 5200 RPM or higher being the red bars all lit. In our cars presently, the red bars all illuminate at anything higher than about 4000 RPM.
I'll post back with results using the new oscillator crystal probably within a week since Mouser has both crystals that I am going to order (5 Mhz & 5.355 Mhz) in stock.
Gonna have to wait until next year for times and/or a dyno session. I've been enjoying the learning process tuning it with the Holley but so far I've rarely gone full throttle. I spent too much money on this setup to blow it up being an impatient idiot. I figured out how to activate the boost-a-pump with the Holley so at least I feel confident I have enough fuel to feed 600HP.
Holley now having the Terminator x setup may make aftermarket EFI tuning affordable to more people. I've looked at the software and it's about identical to the HP/Dominator software.
I'll be waiting to hear what the hp/tq numbers are, but I'm more interested in how quick it runs. Even at 3600lbs it should have enough to get into the 10s in the quarter.
I should be able to hit high 10s, but I don't have a roll bar, so I'll be letting off at the 8th and extrapolating.
In the mean time, you can watch this to satisfy your curiosity. 205s vs my 220 heads, similar sized cam to mine. 611HP
You'll have to wait until 9 minutes in to see the larger cam similar to mine run with the elbow.
Curious if weighed the engine or have a guestimate?
I didn't, but I had the advantage of knowing the old 306 had a 65lb Kenne Bell setup on it (including lower & brackets.) The new setup has a supervic & elbow that's pretty light, so it's about a wash based on what I had read about the Dart block weight compared to 5L.
Gonna have to wait until next year for times and/or a dyno session. I've been enjoying the learning process tuning it with the Holley but so far I've rarely gone full throttle. I spent too much money on this setup to blow it up being an impatient idiot. I figured out how to activate the boost-a-pump with the Holley so at least I feel confident I have enough fuel to feed 600HP.
Holley now having the Terminator x setup may make aftermarket EFI tuning affordable to more people. I've looked at the software and it's about identical to the HP/Dominator software.
Once you can fully lift the bezel, you'll note that there are two multi-wire cables that connect to either side of the dash from the bezel. You'll need to disconnect the 5 pin one (only 4 wires) that connects to the tach (left side.) It's taped so you'll need to cut or remove the tape first. Once that cable is unplugged, fold the bezel back and grab the tach module and lift it. Note the ribbon cable will require you to 'slide right' to remove the tach module cleanly. Fold the bezel back onto the dash and put it somewhere safe. 
Wait! I have this old tube audio frequency generator from like the 1960's! Now I'm definitely looking like a mad scientist. 
I sat down at my laptop and did the math - it should have read 9.2 bars, perfect. So, assuming the micro can sync up to the higher 5.355 MHz (closest frequency available) crystal that I'm ordering, it should read pretty much on the money.