[Tony Curcio]

It sounds like you've isolated everything external to the engine- after 67 posts it's hard to take it all in.

As you said, the valve train is your next suspect. You've had the valve covers off- did you run the engine while they were off? This is a quick way to see if a lobe is flattened, a pushrod bent, a stud pulled out of its threads, or a broken valve spring. You'd probably be able to spot these things even if you just cranked it with the covers off. Any of these will make a popping sound in a consistent rhythm.

An overtight valve just sounds like crap, hard to describe, but a valve is not closing, so it's just weak and shakey.

One more thing I remember chasing long ago- a head gasket that is blown between two cylinders. Spits out the exhaust and intake of the two affected cylinders.

All of the above have a distinctive sound. If it was possible to post an audio file, many here would probably recognize it.

Hope you solve it soon, and hope you'll post it when you do.