Why does a Chevy 350 backfire at high RPM?

Answer 1

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.