I believe the '73-4 B-body differential is 62" at the bolt surface on the drums. I'm not sure on the perch measurements.
It is always better to measure things yourself before making a purchase, or pulling your car apart.
The 8 3/4 rear in this car was a bolt in swap from a '68? (so I was told) C-body, so I think it may be worth your time to get a tape and do some checking...