For pontiacs there is no Big block or small block, its all one block with different bore/strokes.
For the 455s, you can make a big difference with a cam and higher compression, but by the time GM was putting engines over 400 cubes in mid size cars the muscle car erra was all but over. 1970 was the first year you could get a 454, or 455 in a GM A-body(chevelle, 442, GTO, GS)
the mods I listed, could ad up to 50 horspower to a Pontiac, a big jump on a "stock" car.
Chevy did put a Big block in the chevelle, but it was 396 cubes, and dint put out much more horsepower then the other GM motors. Hell Every owner of an original 396 I have talked had one thing in common, lol a thrown rod, while in waranty! lol.