carb backfire edelbrock 1406

[sagacious]

"Something has to be igniting the fuel in the intake or the ignited charge is being allow back through the intake by an open valve. You say it only happens at high RPM, Are you maybe getting some valve float? How old are the heads? How much lift and overlap? Valve Springs? Valve adjusted a little to tight and not sealing?? Just some things to ponder."

lean condition? rev limiter?
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i've questioned the valve float, the heads are as old as the engine, since 1973. they were recently taken off and fel pro valve guide seals were installed. the engine backfired like this at high rpm a short while before the lifters went out, so i know the valves arent adjusted a little too tight. valve springs are stock, could they have just all of a sudden wore out? as far as a rev limiter, i cant find any information on the MSD HEI upgrade module having any kind of rev limiter.. it's an onboard chip for HEI's. as far as the lean condition, i checked all the plugs and they do have that tan/lean color to them, none of them are black.
maybe the 2% lean stock edelbrock #1406 needs to be re calibrated? seems strange the engine would need more fuel though... i run the same carb on a milder performance 350(345hp), crane energizer cam @.450 lift and 9:5 compression.. that engine seems to be getting sufficient fuel and doesnt backfire, but the ignition on that car is just stock HEI with no upgrades other than plug wires. i know that all in all i might have to suck it up and realize the engine is 30 some years old and it could just have a worn cam lobe or weak valve springs. but im trying EVERYTHING i can think of before i resort to that conclusion. btw, the car never blows smoke in any condition and the oil pressure has always been no less than accurate according to the books for a low volume low pressure oil pump. just out of wild curiosity though... the drivers side header is smashed in pretty good on one of the tubes, could that possibly contribute? thanks for all the advice.