Engine is back firing through the exhaust

Timing too far retarded. Timing chain slipped. Bent exhaust valve.

Yeah. I almost lost my 74 Charger to a bent exhaust valve causing a backfire.

Moisture in the cap and wires can also cause backfiring. Check for a defective cap with tracing between the high tension terminals inside the cap.

A backfire through the exhaust can be caused by a few things, either alone or in combination:
1: Incorrect timing
2: Faulty/defective exhaust valve
3: Faulty or defective cap
4: Over-rich condition leaving unburnt fuel in the exhaust
5: Electrical problems causing the wrong plug to be fired. This could be caused by something like an intermittent open in the ground to the coil.