What valve springs are you using? In the past (years ago) I built a Chevy motor (Carb'd) with Vic heads that wasnt making as much HP beyond 5500 where it should have made another 100hp from 5500 - 7000. The heads I bought came with Comp springs that were good to .600 lift. I played with the timing and carb till I was blue in the face and every pass I made I was just getting pissed off. The car wouldnt pull beyond 6000 RPM. My father inlaw recommended that I change valve springs so I went with stiffer springs and changed out the pushrods as well. This was a solid lifter flat tappet cam designed to make power between 3000 - 7000.

Changing the valve springs was like night and day difference. Pulled all the way to 7000. Turned out I was experiencing valve float and didnt know it. Solid lifter flat tappet cams are pretty noisy to begin with and I was not very experienced with them. Live and learn.

Not saying this is your problem. But it is an avenue that you can investigate. Lifters, pushrods and valve springs.

Hurst