Another tip: lose the power brake booster and go with manual disc brakes. I did this when converting my 70 Challenger from a 383 to a Hemi, and I like the brakes a lot better this way. They were way too touchy with the booster. Just use a master cylinder for a '72 Challenger with manual disc brakes to get the smaller piston.

BTW, I designed the Hemi conversion mounting brackets that John Arruzza sells. They should set your engine in the exact same position as the factory Hemi brackets.