I have a 1970 el camino and am in the process of putting the body back on the frame (after painting and new bushings). I notice that the rear bumper does not line up evenly left to right to the body. The drivers side wrap-around seems to be right on the body, whereas the passenger side of the bumper wrap around has lots of space between the bumper and the body. The bumper is mounted on the frame, do I have to try and "Shift" the body on the frame to get the bumper to align? I don't see any obvious adjustment on the bumper brackets.

Yes, you have to shift it over. I loosened all my body bolts and just shoved it over by myself.
I was surprised how easy it went.

Did this very bumper fit on the car before? Has there been any changes at all to the bumper or body?

If the body alignment doesn't work as the last post suggests, maybe the bumper isn't straight.