[joe_padavano]

There are a few differences. The 64-66 cars have different lower spring perches, for example, but you can swap the springs and housings as a unit. The one thing I've always found confusing is that the Type O axle was first offered in mid-year 67 but with the same width as the 68-72 axles. Also, some of the various GM housings use different upper control arm mounting locations on the housing, so you need to swap the housing and upper arms as a set.