1984 Mustang 5.0

Lots of stuff. Cam, EFI, exhaust. Pick up a Haynes manual. It'll show you the physical differences between the cars.

Totally different engine, even the firing order is different I belive. In '85 Ford switched over the the "HO" series and the bottom end swapped over to a roller engine, different heads, cam, induction, brakes, ignition.... I'm pretty sure the rear end isn't as strong either. Don't be discouraged! It's not like the newer parts were all that great, and yours can't be upgraded. Also, your car is over 20 years old, you might want to check out and see what all has been changed by previous owners.

Your car is carbed (unless swapped) so you can build power cheaper. An 8.8 rear end out of a junkyard should go cheap, rebuild it with a better gear and you have a rear end that will last you a long time. If you can, put in five lug axels and do a SN-95 brake conversion, but not really nessasary. Get a crossmemeber out of a 87+ and throw on some longtubes, o/r H or X or PC midpipe, and a catback, and you'll have a good exaust, put on a better carb and intake, get the head ported, a better cam (flat tappit is cheaper) and then you'll be able to rape any stock 87+ without trying.