One sure fire way to turn a 383 into a turd, install a fast rate hydraulic cam. Like PRH said, the lifters will not allow a 383 spin high enough to make use of the cam size. I agree with doing a swap to a solid lifter on the cam to simply check. But the valve springs need to be right too. My 383 is similar to yours. MP 280 cam. It will spin 7000 rpm (not that It should, but it is so much fun). If I had to shift it at 5500, it would be totally neutered..