64 rear end help

Here's some dimensions for your consideration.
63-72 7.25 overall= 53.2" center to center of spring perches = 43.02"
all 8.75 " = 51.4" " = 43"
F body8.25 " = 54.30" " = 44.46"

For my money seeing what i've posted i would use the 8.75 as the spring perch location is very close and the differerence in total length is 1.8" resulting in an approximate 7/8" shorter at each end. Think of it as a mini shortenin job. This should allow you to fit a wider tire in the well without doing anything. Just be sure and allow on the new wheels for diff back space.
Small Block