I had similar issues with my '62 235 in my Bel Air. Seized the rear main 3 times. 4th time we shimmed the rear main, it was fine, but took out the rod bearings on the center 2 cylinders. For what it's worth, mine came from the factory with the rope seal. After the first time we used the neoprene seal. I wanted to keep the 235 but it was too much headache. It's a way better car with the SBC now, even though I'll get attacked for that.
