sometime in the 70s they offered the part throttle kickdown. That's a nice feature, too. Easy enough to add, but they are getting hard to find, so if you find one with it already that solves that issue.
71s and up had the PT kick-down, which in a stockish engine or with a 4bbl, is a nice feature. You don't have to floor it for the trans to kick down, and if you are careful, the secondaries won't open either.
I think the 71s to about 75 are the better cores.
The slanty's got PT kickdown, I think a year sooner, not sure.
