Same here with my three harmonic balancers. Well the second time I thought that the engine sounded funny, but thought it was loose belts slapping around, 10 minutes later the bolt was broken, and I was broken down. On my motor, before I bought it, two pistons were replaced, the #1 and #5. #5 had a bad valve, #1 was loose inside of its walls, although you couldn't tell with the head off. So far its still sitting ina corner waiting for the balancer problem to be resolved. But I may have come across a donor car. BTW if you need a non turbo 300 crank, i got a used one....cheap! and a rebuildable block
Good luck with your hunt, I am curious as to what you find.
03/83 300D 184k
07/73 280 160k
-88 other 153k