So to follow up. I dug into the body some with a wire wheel on an angle grinder. The frame rails and rocker panels seem quite solid. But the sheet metal has at least as much filler in places as expected. Mostly above the rocker panels. If I'm not shooting for perfection, or even necessarily good as far as matching the original shape of the metal, if I just cut all that out and weld new panels to the rockers and wherever the good metal above them starts... that should get things to structurally sound, right? (Along with rail connectors and new floor pans.)