The old bugs really didn't have a frame, so to speak. The chassis pan got most of it's rigidity from a central tunnel structure that ran from the front axle to the engine compartment. The outboard sections of the floor pan had a secondary flange arrangement to tie the lower body to the pans with a multitude of bolts. The outside edges of the pans mainly got their rigidity from the heater tubes built into the lower body and a stiffener below that.
Probably a good idea to take the camper shell off the pan to see exactly what damage(probable rust) you have to the pans. If the center tunnel has buckled or rusted...you're pretty much out of luck. There's a good possibility that who ever did the shell mod, may have cut out the heater tubes in fitting the shell. You may have to built a frame for the shell and not rely on the VW's pans for too much rigidity. To do that, I would think you would have to tie the shell's support into the center tunnel...and geez, I really don't think the tunnel(even if undamaged) would be up to the task.
When I worked for VW(back when the '72s were new
), I cut into many a tunnel to weld new bracket supports for the internal clutch cable tube and that metal is not all that robust. All that said, VW seemed to rely heavily on the tunnel, outside pan flanges, heater tubes and the body to form the car's structural integrity. Any of these component failures will seriously compromise the structure...which it looks like, may be the case. Best of luck with the project.