Misfire at cruise, popping/afterfire at 3000RPM

512Stroker said

Well it has become apparent that your problem is 20 years old that is why the Guy you bought it from parked it.

Some things way off on your leak down readings if you had that much leakage going on you would have other issues. Remove the rocker arms/shafts and start over. Keep the push rods in order and check each one to see if some are bent, just roll them on a clean flat surface.
Does it blow blue smoke out the exhaust and/or out the valve cover breather?

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Haha, I guess if you put it that way.... He never mentioned this specific problem, he said the problem he was having was a very high idle. That issue did surface for me, but has been since been resolved. That's not to say the issue at hand didn't exist as well.

I'm going to conduct another leak down today and will report back with new (and hopefully better) numbers, and while I'm at it, inspect the push rods. I have the stamped steel rockers, and was half expecting to see one of the rods punched through a rocker. Haven't seen that yet, but I still need to pull the driver side cover.

The engine doesn't blow any smoke at all, and doesn't seem to consume oil.