[84LX89GT]

well the throttle body fuel injection sucks, plain and simple. if you modify the vehicle too much it will run like crap because it doesn't adapt well/quick enough and can't measure the actual incoming air.

If you add later model mustang headers you could probably convert V8 mustang h-pipe and cat-back exhaust to fit (stock or aftermarket). The number one thing i'd recommend is gears (like 3.73 or 4.10 to 1 gears). also make sure you have a rear sway bar and getting good rear control arms (upper and lower) will make handing/performance night and day.

Maybe using a dual snorkel air cleaner setup from a GT would help as hell as finding a larger throttle body from a V8 (assuming it flows more air).
An electric fan won't affect drivability and gives you something like 5 hp, also swapping/getting rid of the steel bumpers underneath the front and rear nose covers and replacing them with later model light weight bumpers or getting rid of them sheds ALOT of weight.

just a few things off the top of my head.