A rear end that is 61 inches wheel mount to wheel mount should fit fine. That being said, I got a F**D 9 inch full floater rear end for my 1940 that was 61 inches wheel mount to wheel mount. When I bought the rear I specked out 31 spline axles, a 6 lug on 5.5 bolt pattern, and 12 inch X 1.25 vented rotors. Those large rotors may be overkill on a 1/2 ton, but should be able to stop any load without overheating.
The 9 inch full floater is a great setup. You can get it in any width, or gears you can imagine. It is stronger than a dana 44, and a lot stronger than a gm 10 or 12 bolt.
Around the bay area, rear end work is real expensive. You may be able to find a suitable rear end from a boneyard, but you will still have to Find one that will work, pay a million bucks for it, clean it up, rebuild it, possibly shorten it...
As far as a parking brake... you could either spend a lot of money for calipers with the cables, or you could get a pinion mounted brake which would probably be eazier to adapt to your stock floor lever.
