The backing plates might bolt on without any modifications, meaning you wouldn't have to swap rear ends ... just the brake set up. Rear discs on an otherwise stock 68 rear end. Of course, there are a bunch of other things to change: Master cylinder, proportioning valve, etc.
I don't know about a complete swap. Though I would guess that it's the weaker 7.5" rear end, instead of the more preferred 8.5"
