The base can be different on horseshoe style shifters. For mounting purposes, I've seen some with a flat base and some with a arc on the front edge of the base. I guessing that the ones with flat bases go in Camaros and maybe some thing else. I'm also guessing that the one with the arc on the front edge goes with Chevelles, maybe because of the form of the hump. I wouldn't think that it would matter whether it's a P/G or a TH. Can either type of base be adapted to work on 68-72's? Can the guts from a flat base type be used in the type with the arc? In other words are the guts interchangable?

I don't believe Camaro and Chevelle ones are interchangeable. They follow the floor pan contours. The handles should swap, and the detent pieces should swap too.

probably the same basic shifter with different brackets welded on for the different floor pan contours. GM did this in the 70's with the shifters that fit in Camaros and in A bodies.
one way to know would be to compare on of each side by side.