440 Won't start syndrome

RETARDING timing seldom helps a backfire problem.

A huge vacuum port, like the PCV or power brake fitting unhooked will definately add to problem

You can make some "quickie" tests. Unhook the fuel line, crank the engine with fuel hose into a can, and just look for a reasonable good strong flow.

Any chance the carb is dirty/ gummed up?

Any chance you have a damaged / open/ misadjusted valve?

Might run a compression or better, leakdown test. Leakdown testers are easy to make, just google "homemade leakdown tester"

Low ignition voltage, IE bad coil, damaged coil wire, or rotor or cap can cause backfiring, AS CAN damaged (open) plug wire(s)