Rear end alignment issue 1969 cuda

maybe measure with a plumb bob, and make marks on the concrete, which you can measure directly with a scale. Of course, you need to level the car up front to back , and left to right.
I suppose the rear axle housing could be bent. Probably a long shot. I once bought a 70 340 Cuda parts car, which the PO spun out and backed into power pole at speed. The 8-3/4 housing was bent, along with the driveshaft.