No real reason to go pontiac from olds. It's a pita mostly. Mounts (301 g body mounts, especially the frame pads) are hard to find. I think you can use F body (firebird) mounts, but I'm not sure. With an Olds 403 you can swap in a solid main 350, a 455, or beef up that 403 as olds307 and 403 said. It's more trouble than it's worth in all fairness.
Also, if it has this little doodle on the back of the block on the passenger side, it is either a 326, 350, 421, 428, or a 455. 389s and 400s do not have this lug. Only way to tell from there is measure the bore or casting numbers.