well, it's pretty obvious that the motor is running out of fuel.

Order a #1466 needle and seat kit from Edelbrock. It's 0.110" and will flow more fuel into the bowls. Also order a Edelbrock #1791 electric fuel pump and run 1/2" lines from the tank to the carb. The pump is pre-set at 6 1/2 lbs, so no pressure regulator is necessary. Tee off an electric fuel pressure gauge right where the fuel line enters the carb and monitor it under full power.

http://www.edelbrock.com/automotive_...mps_main.shtml

http://www.edelbrock.com/automotive_...hun_perf.shtml