I think it depends on what you want to do with your truck long term. If your goal is a driver with a really tall first gear - it will be fine. If you want a hauler, you might want to reconsider.
I put a 12 bolt from a 69 chevy pickup under my 1950 truck. I found that the width was within 1/8" and the backing plates accommodate the '53 wheel cylinders as a bolt in so you can retain the original looking brake lines / connections and stock flex line location.
The e-brake cables are an easy modification too - just buy new '69 cables and take the forward ends off and 30 minutes with a lathe and a small press and you have cables that are 1953 on the front and 1969 where they go into the backing plates.
I went with a 3.73 posi and put a mid 80's GM code MY6 4spd with OD of 0.73:1 in 4th gear and connected it to the stock 3-on-the-tree shifter with some garage fabbed linkage. I just stabbed it into the bellhousing - no tail shaft support or cross member was used
It works well and I have a nice low first gear and can cruise at 60 mph at about 1950 rpm. I drove several hundred miles averaging 70+ mph this last summer driving across the US and back - it was a dream to drive.
I have just over 11,000 miles on it now including some fairly aggressive street light drag races against a few buddies with 302 GMCs in their trucks. It is a solid drivetrain and I am very happy with it overall.