Disc Brake swap-A body Rotors

I think thats correct. You are going to have to run adaptors or swap out your rear end axles and drums to the 5" on 4.5". I think swaping out the axles is the best way to go. I bought the Yukon's with green bearings and used 1973 backing plates, hardware and drums to get the bigger 10" X 2.5" brakes. The axles are some 20-30% stronger than stock, and the bigger drums will give you better braking at the rear. You can find thes axles on ebay and many rear end suppliers, even Moroso.