1973 Chevrolet Monte Carlo backfire through exhaust

Put new motor in was told it was a stock 350 but had a bad head gasket. Pulled heads and found it to be punched 30 -40 over. Put new head gaskets in on both sides. Had points dist. Swapped it out for a HEI. Did everything by the book a friend adjusted the valves but they wont stay adjusted and start knocking. Changed plug wires and plugs it idles fine but in gear the higher the RPM the backfire starts popcorning out exhaust on left bank only and falls on its face but yesterday it started popping on both left and right banks after driving through puddle of water. It got louder and louder car died out a few times acted like it wasn't getting fuel wouldn't stay running hard to start loss of power and limped it home I'm stumped please help. I'm pretty good with motors but don't know everything and this is the first I've dealt with this problem motor has a Rochester 2bbl headers stock intake the heads are stock.