Class be seated,
Welcome to Mustang 101
Anyhow, welcome to the muscle side of the street

It won't be long and you'll be walking all over your friends. The first thing you'll notice is that parts are much cheaper for the Mustang than your average import and they'll do more for your car.
There are a few things I'd do for your car:
1. Complete tune... plugs, cap, rotor, wires, coolant, brakes, clutch, what have you. Get access to a timing light and advance your timing a little, then install a 180* thermostat when you are going through the cooling system.
2. Gears, find-out what ratio rear-end your car came with, if you are lucky enough to have 3.08 gears then I'd maybe focus on some other things, then get back to gearing. If you have 2.73's I'd maybe adress this issue a little faster. The way to check... there should be a plaque or tag in the door jamb. This should have a code somewhere on it that says axle or axl and then has a digit code like 15 or R5 etc ('85s had this right guys?). You can use this code on the web to find your ratio, or there should be a code on the axle case or diff cover tag. When you are ready to address gears, You could invest in a full gear-set and labor etc, but I think you would be throwing your money away on the old 7.5" axle... Scour junkyards for a wrecked 86-93 with the 8.8" and swap it out, then gear the stronger axle accordingly. I run 3.73's in my car and love them.
3. Exhaust upgrade, get the true dual exhaust setup.
4. Shifter, pick-up a shifter that feels the way you want, but also has adjustable stops to keep you from ramming it 'litteraly' through the dash. I run the Hurst Pro-comp Chrome shifter because it looks, feels and reminds me of the Muscle cars from yester-years that I love.
5. Tires/structure. "Tires are what wins a race" get a good set of tires and then sure-up the structure of the car with strut-tower brace and subframe connectors.
Those are the basics that everyone has commented on. From there, intake, carb, headers, cam, heads, 331 stroker engine the list goes on, but I warn you, the "mod-bug" will bite and it becomes addicting.
Goodluck man and again welcome to "REAR WHEEL DRIVE"
