In regard to swapping the late C-body disks onto the early C-body, there MIGHT be a way to do it. According to this post in the tech archives, it might be possible to swap the entire front sub-frame from the 75 into a 68.

http://www.moparts.org/Tech/Archive/susp/2.html

I've NOT tried this, or even measured it yet. But I really like the idea of swapping in a later subframe.