Axle conversion?

Backing plates are larger diameter than the "advertised" brake size. You'll need to measure the inside diameter of the drum, not the outside diameter of the backing plate.
You need 10" large bolt pattern backing plates/drums. They can come from any 1965 and up large bolt pattern rear end. Best to get the complete package so you know they match.
You can also use 11" brakes if you'd like to. I have 11" drums from a 74 Satellite on my Demon. Overkill, but I had them available.