I think you’re right - it was the same plant, just with a longer stroke relative to the bore size, and produced about 500 ft./lbs. of torque as a result.
The last Pontiac-specific V8 was the 301 (can't recall whether the turbo or naturally-aspirated version hung in there the longest). IIRC, the car magazines in 80/81 were happy that the 301 was finally a bit lighter than the Chevy 350.
The 301 was a sibling of the 400 and 350 Pontiac engines - all of the "small crank journal" series. The 455 was derived from the 421, a "large bearing journal" engine. I've never seen examples of both side-by-side, so the exteriors of both series' engine blocks may indeed be the same.
What a shame that they're all relegated to history books and car shows. It's tough to believe that Oldsmobile and Pontiac are gone... and equally hard to accept the fact that Buick outlasted them. I guess that name is easier for the Chinese to pronounce.