You can use the MC and booster out of a 94/95 GT Mustang or the MC and booster out of a 94-98 V6 Mustang. The issue with either of these is the booster has a different bolt pattern so you have to drill a couple new holes. Pain in the ass, ask me how I know. The MC is a 1-1/16" bore but they work fine.
So, my suggestion would be to run the '93 Cobra gear as it makes life a lot easier. Either way you go you will need the following:
3 to 2 kit (to take your existing 3 port MC to a 2 port design)
Manual adjustable proportioning valve
Stock combination valve plug kit - used to gut the proportioning side of the combo valve (have to do this to make the manual adjustable proportioning valve work correctly)
Brake adapter fittings - you can use these or build your own lines with the correct metric fittings (pain in the ass)
To to recap, you will need the booster and MC for a '93 Cobra, a 3-2 kit, a manual adjustable proportioning valve, a stock combination valve plug kit, and brake adapter fittings to install the MC with metric threads.
Typically you will raise the rack as most cars are lowered. What I did in lieu of using a bump steer gauge was try and make the inner/outer tie rod combo as parallel to the LCA's as I could with the suspension of the car loaded. If a person does not have a drive on lift you can put jack stands under the rear end and then put them under the "hat" of the rotor with the rotor secured to the wheel bearing/hub assembly with lug nuts. I will say that this is dangerous but I have done this when working on the car without being under it. I do have a second set of jack stands under the rear of the front subframes as close to touching as I can so that in case there is is a failure there is a backup. All I do is get them set up and then remove the jack stands out from under the rotor "hats" and let the secondary set then do the job while I am torquing things up.
The short version is swapping a fox Mustang k member into the 87-8 Tbird can be done but it does not just bolt up and there is a better way, besides.
The long version covers some of the pros and cons of both.
First the swap: Swapping the Fox k into the Tbird requires spacers be used between the subframe and the k where the 4 large bolts, 2 per side, are located right by the top of the spring. I think the spacer thickness req'd is approx 5/8"? or something close. The 4 small rear bolts that also attach the k will probably have to be elongated using an air die grinder with a carbide burr. You will need to do all needed measurements to make sure the k member is properly squared to the vehicle or some weird steering/handling issues may be encountered. All of this is in addition to transferring the steering rack, control arms, and reattaching the brake hard lines that are bolted to the k-member where they cross over from driver side to pass side under the engine.
The better way is to leave the K-member in the car, pull the engine, degrease the K, have a competent welder cut the Tbird engine mount pads off the K, do the same with the Mustang K, line the Mustang pads up and mig weld them in place. Reinstall engine, skip the k install, all the spacer and squaring the K member horse manure, and drive away with the job done right and nothing bufu- rigged.
Consider that the K-member has your vehicles steering, brakes , suspension and engine all bolted to it while you are ripping down the interstate at 80mph, before you start stacking body washers between your k and the subframe.
Then welded all that I could reach with the y-pipe attached to the car. Next up: finish weld, add a couple hangers, and connect to the old muffler.