Tom I have to agree with Ron, the 68 GSS was not a factory built car so I don't think it would be FAST legal, it would be like a YENKO and I don't think those cars are legal.
Grant what is your idea of competitive? The top cars in FAST are in the 9's. many are low 10's. A 69 Big Block A body with a single 4 bbl and it's restrictive drivers side exh. manifold will not even be remotely competitive with the top of the class.
But then again I said a 6 pk wedge couldn't get out of the 12's so someone feel free to prove me wrong again ...
I'll be happy if my 450" 4spd Dart runs into the 11's .