you can add air presure if you still have the throttle body coolant lines,, just cap off the TB end, then put air into the hose you took off.

You could do the same on a heater core hose, just dead end one end and add air to the hose you have hanging off.

i dont know how much air to inject but my gut tells me anywhere around 15psi of presure should start to reveal a air leak noise to help you pinpoint.

you will want to pinch off the radiator kneck bypass tiny line that goes to the over flow tank,,

actually dont go over i guess 19psi which is what i think the stock motorcraft caps were stamped with, i have a few that say 19psi i think.