74 Valiant 318 Backfire when hot

If it is repetitive like a machine gun, look for something keeping an exhaust valve closed. A broken exhaust rocker arm, bent exhaust pushrod, wipe exhaust cam lobe and or lifter. If it only does it on acceleration, then check the initial timing. It likely needs more. Can you get us a video? This IS 2025.
Yeah I can get a video tonight for you. That will probably make this easier