Differential gears and tune will wake these cars up. I believe the power band for these GTs are higher in the RPMs so gears will help you get there faster.
What's the VIN to the GT? I might be able to look up a basic version of it's window sticker.
Other good mods are wheels/tires, suspension, short throw shifter, exhaust, and 1 piece driveshaft.
That will get you started to unlocking the potential of your beast.
Of course, there is a lot more you can do but I wanted to break you in easy.
Since it's a manual, 3.73 gears might do you good. The lower you go (4.10, etc), the sooner you will have to keep shifting.