It sounds like ignition miss to me or the valve springs.
I use to race NHRA stock back when we had to use the original stock valve spring and cam specs, the first motor was a 1968 Street hemi that had 125 lbs. on the seats and 325 Lbs. open with stock lift which was right at .484 lift at the retainers on the intake valves and right at .466 net on the exhaust side, we switch the first motor into a 1970 Challenger with a hydraulic lifter cheater cam and use the same valve spring specs on it, both motors would pull really hard to 7000 RPM with a Hayes S4 ignition box, now known as Stinger ignitions.
I race another NHRA stocker 1970 hemi car with the early Mopar chrome box with the blue heat sink and it would pull hard to 7000 RPM also
My message is to try one thing at a time but in your case I would look at the valve springs first
Did you modify the oil system in the block to oil the rockers full time? If not that is worth doing , Especially on a street motor that gets the oil hot and valve springs hot for long periods of time
I groove the #4 cam journal to do that and restrict the holes size to the rocker arms depending on which rocker arms are on the motor, roller rocker on the shaft get a .040 oil restrictor, non roller rockers on the shaft get no restrictor
On your deal I would look at 225+ lbs. on the seats and 480 Lbs.+ open depending on how much net lift you have. If over .600 net lift look at 580 to 650 Lbs. open pressure and near 250+ lbs. on the seats
you can NOT hurt a steel core solid roller cam with to much valve spring pressure, you can tear up the rollers with to little
If you can see bright stripes on the back side of the outer edge of the cam lobes you don't have enough valve spring pressure