Agreed,
3.73 gears and a good shifter should be your first mod. This way you'll take full advantage of the power you already have.
You'll definately notice a difference in performance...
A K&N air filter is a good first\cheap mod...
Underdrive pullies will help also...
If you go X/H-pipe, be ready to see a little of that low end go away, but you will gain more than you lost in the upper power band....
And unless you want a sound change, I really don't see a reason to use cat-backs on a near stock 99-00 GT, as the stock cat backs aren't very restrictive to begin with...
Near stock mod-motors live for the upper revs, so that is where you'll want to keep it while racing.... the 3.73s will certainly help.
There are some HP and torque curves for 99-00 GTs and Cobras out there somewhere. Take a look at them, this will help you see where you want to be on the tach and while shifting...
These beasts aren't pushrod 5.0s, so you can't drive them as such either...
You've got a great car with lots of potential. A few minor mods and lots of launching practice should get you into the 13.9s on a good day...
Hope this helps
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1998 Mustang GT
13.97@103
"Traction Impaired" or "Nitto Needy"