I'd doubt they're welded so much they could not be ground off the rear and new studs pressed in to the driving flange

Bearings, easy, just replace them.

Unless the bearing has failed massively, the carrier would be fine.

Really though....sounds like you would be easier buying both rear arms entirely off whatever vehicle you decide it's brakes fit you best, and diameter there will depend on what room you have inside the wheels

You haven't stated whether you actually have any discs or calipers yet either ?