Intermittent Backfiring After New Head Gaskets

IMO 6 and 8 would be easy to cross, at the head.
Compression test on the 6 and 8
If lower than the others,
Lash them loose and see what happens

Oh crap, I missed the video
Forget what I said;
I believe you have a valve sticking in the guide, and you gotta fix this asap.
Put a vacuum gauge on it, it will tell you right away. Well maybe not cuz I'm pretty sure that at least one exhaust valve is shooting blanks. But the compression test should show it.
I went back and listened again.
IMO the idle is dead steady so not sure about the valve thing.
Ima thinking that popping could just be accumulated fuel in the exhaust system, catching fire, which shouldn't be there. But the only way to get it to pop is to light it on fire in the presence of oxygen, which should also not be in the exhaust pipe. Sooooooo before I would do anything, I would check the exhaust system for leaks, up close to the engine.
If I'm right
You might be able to/should be able to hear and see that same pressure spike escaping at the point of leakage, whether it be at the head, or at the manifold to pipe interface . If you find it, you still gotta figure out where the fuel is coming from, rather, how it's getting in there. I mean we know where it's coming from, lol. Now, we're looking for missfires and I just don't hear any.
Are your duals full length?
Any cross-over?
IMA Thinking a compression test might still tell something.


What cam is in there?
sounds pretty good whatever it is.

Those FelPros are 039 compressed and 8.6cc IIRC. That hurts a bit, being equal to almost a half a point of compression over the factory originals........ which there's no sense in talking about, cuz you can't get them anymore......