Hmmm, what is it being used for? Drag, Drift, roadcourse? And you can supercharge all of them as I have done.
For drag and driftng, these are fun cars:
1970 Chevelle SS is the most powerful at 842hp/900+torque, weight 1329kg
1971 'Cuda 440 six pack 825hp/905 torque, weight 1282kg
1967 Cougar XR7 754hp/905+torque, weight 1181kg
62 Buick Special is strong also
2010 Camaro SS and "EDGE SPECIAL" (same car) are surprisingly strong. Same can be said for all the newer Vettes, Monaros, GTO 5.7, Vipers and Ford GT's..
Roadcourse:
1969 Camaro Z-28 RM, 560hp and fun car with a decent tune. My favorite old muscle car.
1970 Challenger R/T RM, over 770hp/820 torque... A tune is needed due to the massive power which hurts the handling.
Also, you can go with a new muscle like the 2010 Camaro SS RM which needs a little tuning for the track but is fun!