Welcome to the Team!
Sorry you need a 1967-'70 Impala frame (your series of cars that shares parts were made from 1965-'70). 1970-'76 was the next series of cars, then the car you have (talking about the Buick) belongs to the last production run of B-body cars made from 1977 through 1999.
Your best bet is to buy one from the desert south west as the car and frame beneath it will be rust free. If you need other parts you might want to consider buying a parts car from that area and swapping all of the needed parts that your car will want (betting if the frame is rusted through so is everything below the belt line).
But you are correct GM B-body includes the Pontiac Parisian and Bonneville, The Olds Delta 88 and 98, the Buick Electra 225, and the Cadillac Brougham (the Cadillac limo is a D-body and has an additional 14 inches of car added after the front seat for more leg room for rear seat passengers, but is other wise the same car).
All B-bodies share common parts that interchange; being differentiated only by the engine used beneath the hood. The addition expense to buy another GM marquee is justified by the difference in the motor that was put in the car. Chevy being the entry level was the cheapest to buy and didn't have a big engine installed the way the other BOP cars were built.
The big block Chevy was a truck engine and wasn't as smooth or as economical at low RPM as the Caddy 472, or Caddy 500 nor the 455 used in the Buick, Olds, or Pontiac. BOP big blocks all looked like their respective small blocks; just having larger cylinders and longer strokes to make them bigger on the inside. The Buick 455 is just a little bit bigger than the Chevy small block and was the fastest production car built when the 455 was installed into an A-body Buick Skylark, or Century. It even beat a Chrysler Hemi in a Baracuda body at the drags back in the sixties and early seventies. Though most car mags tried real hard to overlook that fact; as a Buick wasn't considered sporty.
A BBC is considerably bigger and is 140 pounds heavier than the small block; but it is considerably stronger than any of the other BOP engines. But because it was designed with racing in mind, or as a medium duty truck or buss engine, it was designed to run at wide open throttle all day long. This is why it doesn't doesn't make for the best fuel economy even when compared to a larger displacement Cadillac 472.