Ray,
Im starting a VERY similar build as yours with my 55.1
I went with a Ford 9" rear end out of a '69 Ford Galaxy. Its super strong, easy to swap out the gears, and it was posi as an unknown bonus.
You could find new leaf perches to weld onto the rear end that is the 1 3/4" width and would fit your stock leafs, or upgrade to a newer leaf pack and get 2.5" wide perches. It took me an afternoon to sawzall and grind the old perches and coil mounts off the rear end and I will soon be tach-ing on my new ones.
PM me if you have any questions.
