ChryCorp Passenger Car Rear Axle Dimensions and ID: 1962 to 1989
That listing needs to go in the garbage. ol Frank did some pretty crappy research. A hollander manual could have been used, but, I guess not!
A 8.75 flange to flange are 52 5/8. Where that 51.40 number comes from is beyond me.
62-70 B rears ARE NOT the same length for those years.
That moparts link is the best for 8.75 rears.