440 popping through the exhaust...

Check for worn points or incorrect dwell. As the points wear the dwell increases & this retards the ignition timing. In a duel point ignition system it could be just 1 set of points causing it so you'll have to check them both.
Another cause could be if its running too rich and gasses building up in the exhaust & then igniting, maybe by free air getting to the exhaust upstream from a leaking header gasket too.
Dare I say it could be a tight or burnt valve causing it too.(hope not)