Pontiac has no "big block" or "small block". As long as you stay with a Pontiac engine of mid 60s-late 70s vintage, anything from a 301 to a 455 will bolt in. There are a few to avoid, like the 301, but overall it'll be an easy chore if you do change displacements.

To make your 350 2bbl run like a 350 HO, you'll want to swap your carb, intake, heads and cam. For added performance you can update your exhaust to long branch cast manifolds (or headers if you like the noise). While you are at it, consider HEI and internally regulated alternator.

350 HO is a good performer in Firebird.