Old bell housing with a piece welded was lined up. New one was too, where you use the offset dowels and what not? The one with the crack had been in the car since the 80s also and just the past few years the vibration showed up. (Same driveline since the 80s front to back). New one was checked also. New one put in and it was still present, that's why we figured the transmission had to be the vibration( last thing left we didn't rebuild)...