they're called 'BOP' (or sometimes BOPC) housings and they're 6 bolts. the BOPC stands for: Buick Oldsmobile Pontiac Cadillac basically, the 6 bolt housing fits everything GM except chevy and gmc. the chevy housing is 5 bolts and has one bolt at the top in the center.
The Chevy bellhousing is still 6 bolts however there is a pointed top on the bellhousing(overall shape). The modern 2004R auto trans has dual bolt-patterns, but it is an exception. I wish there would have been more of the dual setups.
just make sure that the bell houseing has daul nose cones , pont and old starter is on the other side from buicks
