Hey minipete, sounds like you have been quite thorough, not being familiar with a toploader myself, have a couple of general ideas, does the bell housing have an inspection cover so you can see the flywheel and clutch, and see if something has come lose or broken, does a flywheel have a counter weight like a flexplate? The pressure plate assembly will rotate regardless of pedal position and any imbalance in this will effect the engine,
Curious to see what you come up with, good luck, Andrew..,