IF it were a monte frame, it would have coil spring pockets in the rear as part of the frame. If someone changed it to leaf springs, the sag is probably due either mismatched springs OR improper installation. If the frame is not a monte, it could have been tweaked in a wreck as posted earlier.

A monte frame is a bit wide for an old car.....was the frame narrowed in the back? Did they get that part straight? Unless they had a surface plate/jig, it would be possible to have it warp if they narrowed the frame.