One of the problem areas we had with the GMC V6 line was distributor wear. Can't recall about timing chains, though. They will run forever if you keep your foot out of the carb and the rpm's down. Crank is not well-balanced and the rods will stretch. I would be pulling that engine and rebuiling it, rust in a cylinder is not good, and it does sound like the timing chain is stretched. Is it possible that your rubbing is coming from the oil pan dent interfering with the crank?
