When I went to an open drive line in my '51 I tried a 62" wide (mount surface to mount surface) from under a caprice. Since the new diff was 5 bolt I used newer steel rims. Since I was going with wider tires, I needed wider rims too. So, when I slapped the tires on the new diff they hit the frame.
That is when I figured out the backspacing on the newer rims ain't the same as the older ones, plus my rims and tires were wider than the stock ones.
See, the mounting surface on the stock tires was smack in the middle of the rim, so the distance from the mounting surface to the frame-side edge of the rim was like 3" or less. With wider tires you need wider rims. Plus, newer rims have the mount surface about 3/4 of the way to the outside of the rim. So, with my 7.5" rims, 3/4 of the width was on the frame side of the mounting surface.
I ended up using a 65" wide rear end. (from backing plate to backing plate.)
So, to sort of answer your question, no one but you can figure out how wide a rear axel you need because only you knows the backspacing and width of your new tires. With stock tires there is room for an extra 1 1/2" on either side so a 65" diff should be okay. With wider rims, or modern rims, a 62" diff might be a problem.
