Two threads on the same page within a day about the same subject. Please, just bump your old thread.
You do not need to change the computers. The 1990 AOD computer is what you want to run. However, you will need the reverse wiring pigtail from a 1986+ Mustang. The 1985 mustang and older wiring harness will not plug into the newer body hardness.
Swap the entire pedal assembly with the brake pedal and clutch pedal in. It should work.
1984 is a non-wc t-5. Take gear old. Also weak. You sure you want to run this? Uses a 10" clutch and smaller bellhousing as well. It also won't have the correct double hump x-member so you'll need to mod the one in the 1990.
Like said before, in terms of mechanical parts....1986+ is the way to go