[Marshall V. Daut]

Good questions and something to consider, but Dkrahn states that the noise is a DEEP loud knock. That tells me he thinks it's coming from down deeper inside the engine, not up top in the cylinder head area, assuming he was able to isolate where the sound was coming from. If a large bore piston was hitting a head gasket's cylinder edge, the sound would be higher up, softer and not a deep knocking. I think we need to wait until Dkrahn has had a chance to short out cylinders to tell whether the knock is from a connecting rod or main bearing. If the knock continues without change while doing this, then pistons hitting the head gasket might be re-considered. That noise would be audible whether a cylinder fires or not.
'Love these Model A mysteries!!! Especially when the cause is uncovered and shared with us.
Marshall