"REAL shade tree guys....fall in behind me"!!!!!
Next time you change oil...before you drain old oil dump in a quart of transmission fluid and let it  idle in the driveway for about 10/15 minutes. Then drain immediately. High detergent in the A/T fluid will/should break loose any funk/junk/sludge that might have accumulated.
Worked for me several times.
Never was as brave as my Dad.......he usta flush with 1/2 kerosene 1/2 oil!
6sal6

I made a few observations yesterday while taking another crack at it. TimC asked if it pops through the carb, it does not. I have dual exhaust with no crossover pipe, boths pipes pop. I did another cylinder cut out test, the popping never stopped.

I can't isolate an offending cylinder and I'm thinking the culprit is somewhere in the ignition system. Seems plausible that the vacuum needle dropping back could be unburned fuel igniting in the exhaust manifold, pushing back on the engine on the exhaust stroke.

The rotor button isn't fitting snug on the distributor and there was evidence of arcing so I'll start by replacing the cap and rotor. I have a new Pertronix I and flamethrower coil and run a ballast resistor, so coupled with my old wires and tired charging system I might have to little voltage feeding the coil at idle. I'll try jumping 12v to the coil and bypass tje resistor to see if it clears up.

MS, I replaced the PCV valve, check out the original '69  Autolite that was still in service-