From the OP's suggestions, I'd go with a 70-74 B'Cuda as it has the shortest wheelbase (108"), and probably the lightest overall weight, and absolutely stay with a small block V8 engine for light weight up front. Search for 318 car for lower initial purchase cost.

Mopar Mitch "Road racers and autocrossers go in deeper and come out harder!"... and rain never stops us from having fun with our cars... in fact, it makes us better drivers! Check out MOPAR ACTION MAGAZINE, August 2006 issue for feature article and specs on my autocross T/A!