Where do I start....?

Probably going to be blasphemy here, but have you thought about a Ford 9" rear end? Readily available and pretty normal swap in for non Ford cars.

Do you have the 360 drive train at hand, or is this still something that is a future purchase? If you don't yet then I would do a light rebuild on the 225 (new gaskets and cleaning) and get that into the car as a DD until you can get all the parts you need to do the v8 drivetrain swap. Personally I'd go ahead and send the car to the body shop and get it painted and patched first then do the interior. If you've purchased the drivetrain by the time you finished that throw that in. I've done things in different orders and this way has always worked out the best.