Yeah - I corrected some of that last year when I put steel Jesel rockers in and custom Trend pushrods. I pulled all 4 VC’s last weekend when I first had the problem and checked everything. Didn’t see any issues. That was my first thought because I ran it several times last year on 7 cylinders!! I must have missed tightening down a rocker to a stand and it worked loose! Pushrod came out and ended up in the lifter valley unharmed!! I have no idea how I got away with that, but bought fresh rocker and reassembled. Was able to pull the pushrod out of a oil drain notch in the head!! Fiasco.

Broken valve spring? Rockers, burnt valve? Leak down?
One of the only times I had backfire through carb was when I have a rocker stud pulling out / loosening exhaust side.
Just throwing out ideas.