Don't know if this will help but here is a little tip that can take the guesswork away if you are considering installing a donor rear end.
Take a stick and make it the exact inside measure. brake drum to brake drum of the original rear end. This makes for a very precise gauge while going to a junk yard to check sizes. Its quick and accurate. In my case I found a 1971 Olds rear end and the fit was within 3/4" of the original & it looks stock under my truck.
Hope this can be of help to you.