Sounds like the valve train is not the problem
The further soot travels up the intake manifold from the head, the worse the reversion problem.
But, your problem gets worse at higher rpm? and reversion usually decreases as RPM increases
That's very curious to me
Can you give us a run down on the engine mods especially head and valve work and cam degree install, spring pressures etc
How well your manifold controls reversion pulses and how well it is able to equally distribute fuel and maintain fuel and air flow velocity will determine how large the carb can be and how much power you can make. You can put a band-aid on it with different spacers like sheer or merge or anti reversion plates but that's still only blocking the pulse from reaching the carb the reversion pulse is still there it takes the proper intake or the correct cam to cure the problem!
I'm still curious why the reversion is at the upper RPM range your intake charge and exhaust should be at peek velocity and negating a reversion pulse
My thought that my curiosity
draws me to is
: If the intake valve is closing on time at lower rpm but slower (late) at higher rpm that would cause a higher rpm reversion pulse!
(intake lifter loft, intake spring pressures, intakes bouncing off seats?????
It really puzzles me LOL