Why does a Chevy 350 backfire at high RPM?

AnswerYou don't say what ignition if points could be gap setting.

If HEI, pull dist cap off pull rotor, turn it over and see if there is a a black burn mark on bottom if there is the rotor is bad.

Also if carburated the carb could be going bad and backfire can be caused by lean burn , ( not enough fuel)

could be sticky valve, or wrong valve clearance, broken valve spring

You might also lubricate the centrifugal advance counter weight pins, use a small amount of white lubricate on the pins, make sure that the pins are not worn, check for sticking.