It's an 8 3/8"- pretty much the same as an 8 1/4" so you can get one that will bolt right in out of a 73-76 A body.
The rear in the R/T is built pretty crappy, I switched to a 9 1/4 from a regular 5.9 4x4 Durango in my wifes old R/T when the pinion bearing started making noise.