Mount it to the bumper locations. With the tub stripped, it will be fine.
If you have lots of rust in the longs (the big butt test causes flex) then you want to brace the door openings for added support.
My car is a 76, so I have the later bumper mounts. As you can see, I just drilled a hole through my rotesserie cross member and bolted it right to the bumper mounts. On an early car, you have to fab arms and a mounting plate. No big deal.
Zach