i used stock 92 headers on my 82 GT and my 79 coupe. I added 2 1/2" cat back dynomax on my 82 and it worked fine. The 79 I had pro chamber H/X pipe with dumps with flowmasters. Worked great. You can use a double hump cross member off any 5.0 from 86 to 93 or you can use the double hump off a 91-93 4 banger...........I have one on my 1991 GT right now and it works fine.
E7 heads work well with a little porting, and ok box stock. GT40 P's work better but have exhaust manifold to spark plug issues that have to be addressed.
85-87 has same came in 5.0's as does the 88-89's and also the 90 thru 93 has same! all can be made to run in the twelves with proper suspension, fuel etc....
Good luck
Jack
All the externals from any year FOX 5.0 will bolt on and all interchange with minimal work if any. As far as cam and heads go it is really just a matter of taste. I'm not sure what you are looking to do but I have just ported the stock heads and used comp cams before and been happy.
im running a set of ported /polished gt40p heads on my 85 with a holley street avenger 670,mac shortys,bbk x,performer intake ,,had to switch to a double hump crossmember,all parts interchange with no probs.just make sure to use a 86 up flywheel and clutch or you will fry that stock 10",i didnt think i was adding that much power until i fried mine and got scolded here for using the 10"lol. ive had several fuelies,carb setup has really worked good for me with no drivability probs,the 4barrel is a blast to drive,plus its alot cheaper to build up than a fuelie,good luck
If you are going to keep the C4, you will have to shorten the tubes on the double hump to get it to fit in the frame brackets. If you swap to a T-5 or an AOD, you will either have to cut and re-weld the rear bracket, or modify the crossmember to get everything to line up. The reason is, the brackets 0n '79-'81 cars are only 2 1/2" in width, while the '82 and up brackets are 4 1/4" in width.
I had to go through this with my '79 to mount an AOD and dual exhaust.