62-74 A body, 62-72 B body, 70-74 E body will all bolt on, but the A body arms are longer, like pointed out in your first post.
The bigger spindles in the kit car applciation was becasue they were heavy duty enough for short track racing, but weren't huge by large 3/4 ton truck parts. They also used the thicker 1.25 rotors which make better heat sinks in shor track racing. 35 years ago there was no baer, Wilwood, or other to make trick racing brakes. Everything was sources from trucks.