If the structural components of a unibody shell are sound and undisturbed I don’t see how any body flex taking place from being mounted up on a rotisserie could become permanent over any length of time. If it were so weak as to permanently distort under its own weight these cars would have folded up long ago. With that said, full quarter panel skins are considered a structural component on these cars and should not removed or replaced mounted up on a rotisserie. IMO, short of having the shell secured to a frame table or jig, the only safe/accurate way to change the skins would be, as mentioned, having the body supported at suspension mounting points. Not just the forward spring hanger mounting but resting on mounted springs distributing the weight across both front and rear spring mounting points.