I have a 73 455 olds what alternator do I need or part number for replacement nobody can seem to figure it out

Probably just a gm alternator.
Being a '73,  It could have a external regulator.

If it's missing, I'd put a internally regulated  gm alternator, available at any parts house.

If enclosed engine, get a marine gm alternator.

42A is standard. Boats don't really need more than that. Unless you have a crazy huge stereo system.

Dan'l