Are you talking about John Buscema's Cobra? He's been running in the Silver State Classic for years... there were a couple write-ups on him in MMFF or 5.0 Mustang a couple years back, I can't remember which. But he took the cup with fairly few mods for a guy running in the unlimited MPH class. What got me was that he was running 4.10 gears in that car... it must really love RPM!!!
I really don't know much about aerodynamics... just a VERY simple physics background... but I would think that three areas would need to be targeted:
1) Front chin
2) Rear underbelly
3) Rear wing
The front would need a bigger chin to increase downforce at the front wheels... also, all openings in the front should be sealed to create less drag (e.g. fog light ports, grille, etc.)
The rear underbelly (if that's even the correct term) would need to be smoothed out. With the GT, and even the LX bumper, you got to be pocketing a ton of air under there around the gas tank.. I'm kinda envisioning a smooth Fiberglass panel that covers that whole space over the gas tank and is integral with the rear bumper
Lastly the rear wing, obviously for increasing downforce at the driving wheels.
Guys please feel free to jump in and correct me if I'm wrong, like I said, I have no formal background in aerodynamics whatsoever, this is just what simple intuition tells me.
Oh yeah, one last thing... Balls!
-John