Misfire at cruise, popping/afterfire at 3000RPM

Exhaust valve is open when that happens.
Does it mean...
A. the valve isn't closing
B.The spring pressures arent right, again, leading to A.
Again what camshaft and what spring part number and what pressures did they tell you they set them at?

If it was loading up on fuel it would eventually blow the Muffler case.. that wouldn't cut out like lean...it would blubber the tone would get real low
I have never known what cam I have. Previous owner didn't have any documentation on the engine build, so when I sent the heads off to the machine shop, I measured lift. It was something like .447 on both intake and exhaust -- it's written down somewhere -- so they did their best to match the springs to that. I'll try to dig up the spring p/n, but the pressures they gave me were 120 seat and 285 open. I know that probably doesn't help without knowing the spring part numbers.

One thing I didn't do once the heads were back and installed was to perform another leakdown. I need to do that.

Have you put a timing lite on it? Check the strobe action. Make sure the advancing is smooth on the balancer with NO missed strobes, No jumping back and forth of the marks, and No skipped strobes. If any of this happens, and you have a magnetic pick-up in the distributor, then the polarity of it is reversed. Flip the wires, reset your timing, verify you strobe-action is now correct, and roadtest it again.
I know you've mentioned reversed polarity before, and I've tried to pay attention to the light while checking timing. The light seems pretty consistent until it gets to the higher RPM, and then it will skip around a little in position, maybe miss a strobe here or there.

My question is, how would polarity become reversed? New distributor, I've tried 2 additional harnesses for a total of 3. Are you referring to having the coil wired backwards? Initially I thought you were referring to the distributor pickup wires, but how would I end up with 3 harnesses and 2 distributors, all with incorrect polarity?