Congrats on your excellent purchase. I just bought an '89 GT the 24th of last month. Not much i can add to Mr 5 0's post, i agree those are good things to start with. Check the right fender well to see if the air silencer is in there, if it is, rip it out. I heard that is worth 2-3 horsepower and it's free. If you're not concerned about emissions testing you can gut the catalytic converters with a prybar or other long sharp instrument. That's worth i'm guessing 5-10 horsepower and that's also free. If you gut the cats you can run a short belt to bypass the smog pump and that will give you up to another 5 horsepower i believe. A 180 degree thermostat will keep your engine cooler. stock is like 195* and you can get 160* thermostats but i've heard that's too cold for most engines, so with a 180* stat your engine will (theoretically) last longer and i've heard you can gain 1-2 horsepower by doing that and it's only like $12. one thing that i did when i first got my '84 was to drain and refill the manual trans fluid (regular Mercon ATF)and i found some nasty stuff in there, it improved shifting ALOT as well. Good luck, and again congrats on your excellent purchase
------------------
previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT