Sparky,
Just for $hits and grins do you know what the weight was on the rear axle of your old car? As I remember it was also a flat bottom car, did you run it with some rake? Ever dyno the engine? My guess is a 500+ inch BBC on gas probably is good for 1100 to 1200 hp depending on displacement and rpms.
Your two timing slips probably have a lot to say regarding total available horse power, aero i.e. frontal area x coefficient of drag plus rolling resistance plus skin drag. Your old car looks like it could probably run right at the 310 mark with the hp you had at the time regardless of the length of the course. Definitely against the "aero wall".
Rex