Mr. P body has a good point. My car had the outside structure of the inner quarters left but the whole center was cut out and the back windows bolted in place. That way it still had some structural strength since it tied the door jamb back to the package tray, and down to the rocker. It also left places to attach the interior panel and hide it all.

The problem with my car, is that they did all that to save weight, then stick welded an old bar in it that weighed about as much as I do. I guess that's the way things were in the 70s.