340 8 bolt crank info wanted

If you pull a spark plug, can you get a bamboo skewer down the hole straight enough to measure the up/down travel of the piston? Might give you a ballpark idea of stroke but it sounds like the stroke isn't the issue and if the clutch parts check out, you might be getting up close and personal with the crank. Drop the pan first to see if anything is loose or wonky on the crank weights?