[Daves55Sedan]

Some possibilities:
If rear leaf springs from two different year cars are used, it is possible that the center pins in the leaf springs (which mate with the spring perches on the rear axle housing) may be in different locations relative to the ends of the springs. That would cause the rear axle housing to be mounted off-perpendicular to the frame side rails. In my neck of the woods, this is known as "dog-tracking" down the road. In extreme cases, it is often impossible to obtain a good front end alignment because the rear wheels are aiming so far off of parallel with respect to the front wheels.
A major corner impact, knocking the frame out of square (re-shaping it into a parallelogram) would have the exact symptoms as described in the mis-matched leaf springs, but it would have had to have been a very bad collision. However, it only takes a couple of degrees to throw square off to the point that a good front end alignment cannot be achieved.
Body to frame mounting may be off somewhat, but you should be able to tell by noticing problems with hood misalignment and the two front air deflector panels on each side of the radiator are both slid as far as they can be in one direction in attempt to keep centered with the radiator support.