70 duster 8 3/4 conversion to 4.5 drum or disk hardware

Hey guys thanks, I was looking at getting these axles from classic industries, https://www.classicindustries.com/mopar/parts/mm3042.html, but I want to make sure that I understand that if I do that, then I can't use the backing plate that's currently on my rear , I would have to get some from a donor such as a B-body or a M-Body diplomat? Thanks guys