[grumpy]

M41,
I doubt if it's PISTON knock, but a 331 rattles a bit to start with, those are very oversquare design plants with VERY SHORT piston skirts, good for 12,000 of street driving, at that point it might become unbuttoned skirt rattle. I would suspect it's more in the area of cam overlap/timing issue, not clearances.
grumpy