I don't know whether or not full-size Oldsmobile or Pontiac third members would interchange. My '64 GP has the sturdy 9.3" ring gear. https://www.hotrod.com/articles/1303sr-1957-1964-pontiac-and-oldsmobile-rearends/ This article does not mention Buick, so I'm thinking they may not interchange. The only way I know to be sure would be to find someone with a vintage Hollander Interchange Manual. Those books were used by auto recyclers (aka: junk yards) back then to identify car parts that would interchange between makes, models or year of manufacture.