My G had a knock last year. I tried the flywheel, then the clutch driver. I had just overhauled it a few years earlier so I knew the pistons, wrist pins, bore job, and head, were all up to snuff. Ended up being a rod bearing. Had the crank turned and she's fine now. I learned there a lots of possible causes for a knock, ha. Seems to me, I heard the saying before that if you tighten up only the top end of an engine, the bottom end will go out quicker. That would make sense on my tractor. Wonder if your 70's crank was ever turned?
Your post brought back bad memories! Good luck.