Ummmm....Buicks?
Seriously though, the BOP (Buick ,Olds, Pontiac & Caddillac) bellhousing transmissions are most common because they were used in so many division's automobiles.
Most of the older (1965-late '70s) vehicles from the aforementioned divisions had TH-400 transmissions when equipped with a motor 400+ cubic inches. The smaller cars (A & F bodies) will have the short, more desireable cores as opposed to the long versions used in big four-doors and wagons.
