[whitetop]

mr 70
Not 100% sure about this but this answer is probably pretty close. Weight & Size. Unlike the Super Stock ranks which were broken out by class, the F/C class in the '68-'72 time period I believe just ran by a minimum weight requirement(I'll say 1800 lbs for argument sake)) so why disadvantage yourself by running a heavy 2200 lb '70 chevelle barge when you can run a small Vega or sleek Camaro weighing 1800 lbs and still be legal? Also the earlier Chevelles while called F/C were that in name only-still looked stock bodied in a way. They were very unlike the later flip top Funnies.
Dave