360 running on 4 cylinders

IMO, it has to be connected to the dual plane, or to the right half of the carb. Or to anything connected to that side of the intake.
Like maybe;
a ruptured booster Diaphragm?
Or the floorjet is sucking air.
or the intake plenum, on that side, is stuffed full of rags,lol.
IDK

I just cannot see it being related to anything else. I doubt you have an intake leak at BOTH heads., and I doubt both carbs have the same problem, that is just too coincidental.

But there is one thing; I have seen plug insulators crack, and fire fine in air, but not when screwed in.But I can't figure why 4 would puke at the same time, on the same plenum, while cruising. That would be the ultimate coincidence,lol. Still, I would swap them around just because.

I'm assuming your compression test was done cold, or warm, but not HOT while in fail-mode; so IMO, it means very little. However, I'm not thinking your issue is compression related; the only thing that ties those particular 4 cylinders together, is the dual-plane plenum.