ChryCorp Passenger Car Rear Axle Dimensions and ID: 1962 to 1989
Both numbers are accurate, just to different points.
Mine (57 1/8") is from out of axle flange to axle flange, with drums. Or, inside to inside of wheel mounting surface.
Crackbacked's number (52 5/8") is housing flange to housing flange, and is also correct.