probably a dr diff question. or andyf.

I don't know b-bodies that well.
I know for c-bodies, which doesn't exactly help, that in 69 the spindles changed.
everything bolts up from 65-73, suspension wise. whole setups will work.
but you can't swap individual parts between them.
Like the rotors don't work between certain years. 68 won't work on a 71 etc..

I imagine part of it is the budd disc system that went away in 68.

The other question would be are you sure it is a factory disc system and not an aftermarket?