[84LX89GT]

In terms of things you can do in your yard there's not a whole lot that will make much of a difference.

You can get dual exhaust conversions fairly cheap and you can get take off mustang GT exhaust for very reasonable prices. That's really easy to install.

I suppose a cold air pipe would help a tiny bit, also some underdrive pulleys will free up a few horsepower.

If i were you there are two things that would be a MUST even though they may cost a little more:
rear-end gears (4.10:1 for an automatic, 3.73:1 for a manual transmission)
and also a higher stall torque converter (if you have an auto), which many manufacturers have support staff ready to recommend which one is right for you. Both of these aren't too hard to do, but require knowledge of what you're doing to do it. Also they won't effect the way your car runs, although they may drop mileage down a little bit (not a whole lot though).

Also if you feel like it maybe a JET or POWERCHIPS computer chip may add a few horsepower as well.

Basically i would say GET THE GEARS, they make a huge difference in the feel of any car. If you have an auto, the higher stall converter helps put the engine in the power band faster and pulls the car where it makes torque instead of lagging behind.