Why would a 350 Chevy exhaust backfire?

If there is a exhaust leak anywhere in the exhaust system it will make it back fire out the exhaust. Also raw fuel being in the exhaust pipes will make it back fire too.

That is if it is back firing when you let off of the throttle. But if it is back firing when you give it gas that could be bad plug wires are spark plugs, Timing may be to slow. Cam lobe worn down, Not getting enough fuel. could be a # of things. Check these things first, and if you still have a problem, then ask your question again.