Wheel base difference side to side

You might also try taking a measurement from the body front perch (where the rear spring mounts) to the center of the axle tube. If the car has been owned by someone else and they replaced or moved the rear end they could have put the rear end back in incorrectly. The front of the rear spring mounts against a vertical flat surface which make it easy to measure back to the center of the axle tube. I would think that these vertical flat surfaces are datum points (locating points for the suspension). Unless the car has been in a major accident these spring mount points should be a good place to measure from. When I replaced my rear end I used this measurement to get the rear end perpendicular to the longitudinal axis of the body. When I took the car to get aligned I asked the alignment guy to tell me if the rear end was parallel to the front wheels (axis). He said that it was within 1/64 of an inch, so I guess I did a pretty good job of aligning my rear axle!

Some people elongate the slot in the spring perch (on the axle tube) to move the rear end forward or backwards. If you don't measure from the front perch mount surface there's no way you can get the rear axle perpendicular to the longitudinal axis of the body.

GOOD LUCK!!
treblig