I would suggest to measure the car from LF lower ball joint to RR spring index head, and RF lower ball joint to LR spring index head. This is cross measurement to see if car is straight. There should be no more than 1/2 inch difference between the two measurements. Then I'd measure from qtr panel to spring index head to see if qtr's are straight, car could have been wrecked earlier in its life and repaired poorly.
As mentioned earlier, check rear end code, right year? or a replacement? Spring pads asysmetrical? Rear frame rails ever been replaced correctly? Is driveshaft parallel in tunnel? Both axle shafts the same length and the same style?
Good Luck, Everett 68/350/PG/11.90/115mph