[84LX89GT]

I think MAC may make a bolt on system for your car and for the most power, eliminate the stock y-pipe and single exhaust, this will also make it sound more similar to a V8. They also make pulleys, but ASP (auto specialties performance - i think their name is) probably also makes pulleys for your car. They can advise you what would be best on the street (you'll also have to realize you can't always have all your excessories on at once at a stop with your lights on...it'll drain the battery pretty fast).

Exhaust wise, you can get h-pipes (offroad and with cats) that go from the headers back (you can choose whether you want long tube or shorty) and will fit up to GT 2 1/2" aftermarket exhaust, which will give you ALOT of exhaust options to choose from.

Other things i can think of off the top of my head for your year stang is a supercharger. You can use '99+ intake manifold/heads/throttle body (or aftermarket TB) which will increase it from your car's present 140 something HP to about 190-200, which would be money well spent. If you adapted to the later model intake manifold/heads/induction, etc. You would also have the needed airflow to get alot more power out of a supercharger.

Good luck, hope this helps.