Buick, olds and pontiac (commonly shortened to BOP) had one bellhousing pattern, and chevy has a different one. There's also a BOP specific rear end and a chevy rear, and a BOP specific carburetor, and a chevy one, etc etc. but that's referencing different companies doing different things
Different engines was done as a seperate identity thing, like not producing a mopar with a ford motor they werent cranking out pontiac tempests with small block chevies.