Welcome to StangNet. The O2 sensors help the car fin-tune the air/fuel mixture for gas efficiency. When you slam the pedal to the floor (Wide Open Throttle, or WOT), the car's computer ignores the O2's and just dumps in fuel. Without O2 sensors, the car will run in a 'limp' mode where it just dumps in fuel all the time. A well-running GT should get between 15-17 city and at least 20 freeway, depending on your driving style. I highly recommend buying O2 sensors to get the engine running right.
Torching the springs is usually a bad idea. Shorter springs need more strength to handle the lower height, or you'll keep bottoming out. Most aftermarket springs are also progressive rate, which means they easily move a little bit (to absorb small bumps) but get stiffer the more they compress (to handle fast turns).
Someone else will have to pitch in on the tranny, though I bet you someone will suggest you switch to a manual transmission.
No one sells the special Mach 460 speakers new, you'll have to buy them used. I'm sure you'll get some offers from folks selling theirs. Be aware that the stock Mach 460 speakers are not normal speakers, they're bot ha mid-range and a woofer. Replace them with standard speakers and you lose all your bass.
I'd also recommend adding in sound insulation in the doors, and pulling the OEM radio for an aftermarket one. The OEM radio doesn't give out enough bass and most folks think it just sucks. Put in an aftermarket radio and the Mach 460 will wake up a lot.
