You could try measuring from the inner mouting point of the lower control arm to the ground, and then from someplace on the outer point of the control arm to the ground. Do the same for both sides, makeing sure to measure in the same places on the arm, compare. That should make sure your finder isnt just hung a little funny or something like that. It's entirely possible the more I think about it, but I'd double check.
I replaced the front springs on the 74 when I rebuilt the rear end mostly just to have matching F41 spec springs since I had stuck one in the rear. I didnt notice any sag really, it was a babied 120K mile car, driven by a woman. I still have those stock springs if your interisted in them, though it would be a bit of a crap shoot as to weather they would be any sagging.