Just going from memory, so I'm not 100% sure on this.... The B-O-P compacts shared some sheet metal for '61 and '62 - doors, roof structure, glass, The Buick and Olds also shared the floor pan. The Tempest didn't share that part because of its rear transaxle. The bodies all had a refresh for the 1963 model year, so the sharing didn't occur as much. I know a bit about these because my first car was a 1963 Olds F-85 Cutlass post coupe, like this one: