If I was starting off with a pretty much unmolested GT and was looking to get blown in the relative near future, I would invest in a set of heads, cam, maybe a P-51 or Riechard intake, Mass Air Meter(80mm or bigger) long tubes, and a good flowing muffler. Gears---everybody is different. I run 4.10's in mine as a daily driver and still see 22-23mpg. Granted I drive like grandpa to get that around town.
You will deffinately need a good tune, especially if you get half of the stuff I just mentioned. Good thing now days is that you can reflash your chip/ computer as many times as needed, and while on the dyno. (Not sure what your level of experiance is with tuning) Anyway, you are certainly on the right track with this car. If you need any advice, or want to order something feel free to give myself or any other reputable shop a call.