take off the valve covers,borrow a dial indicator if you do not have one. Mount so that you can measure valve lift at the rocker.It sounds like a mild performance street cam. Some of the 383 crate motors were very high in torque and moderate in horse power. Better heads and cam will likely net 75 plus horse power,though its hard to say without knowing exactly what you have.
rolling off on the top end might be not enough fuel for that engine at max RPMs,borrow a fuel pressure guage and run it through the gears and see if pressure drops near the top of the gears