What oil pump can you put in a 1978 Buick lesabre custom with a Chevy 427 cubic inch big block motor?

A Chevy big block uses a Chevy big block oil pump regardless of what kind of car it's in.

A standard volume (original equipment) oil pump for a big block Chevy is a Melling M77:


A high volume oil pump for a big block Chevy is a Melling M77HV:


TRW, Federal Mogul, Wolverine, Speed Pro/Sealed Power, etc. all have their own part numbers for basically the same pumps as listed above.