If you already have an 8 3/4 rear, you only need a sure grip in the differential. I'd just cut out the rust and patch it, POR 15 it and be done, but get all your mechanicals complete first. It takes a long time to convert to a new hemi and your new paint will be trashed by the time you get it on the road, ask me how I know....
Good luck, and keep posting questions as they come up.