If the car has headers that are more suited towards the 800hp motor, like what is typical on a dragster these days(big tubes, short primary length, short collectors or mufflers), then that would reinforce my belief that it's the shape of the power curve, and the motors unwillingness to pull itself out of the big dip in TQ when the converter starts to put up a fight.
If it were a typical street/strip type chassis, I'd say add about 18"-24" of collector length and see what affect that had.

All of this assumes that in the end you rule out the carb & ignition as being the problem.