Dwayne, you called that one pretty close.

542 CID, EZ-1 Heads
717.7 hp at 6000 rpm, 677.1 ft-lb at 4500 rpm, 2-2 1/8 X 4 dyno headers (stepped)
678.0 hp at 5900 rpm, 673.3 ft-lb at 4500 rpm, Hooker 1 7/8" fenderwell headers(long)

39.7 hp difference. Time for some new car headers

I bet even the Hooker A-body 2" fenderwells (~43" long) would be 30 hp short of the 2 to 2-1/8 stepped headers.

One more thing, with all that torque at 4500 rpm, it might push the 8" converter stall and shift recovery rpm higher than he expects.