It looks like the frame/mount in the photo has already had some welding done on it.. probably a patch plate. The welds don't look right and the frame tube is too flat at the corners. You may have a shat sandwich here.
I wouldn't drive it on the road if the body mounts are shot, and if you have one rotted like the photo then there's likely alot more. The ones that are just spinning are probably doing so because the captive nuts inside the frame have come loose. I corrected all this on my replacement tub when I swapped it, and several mount bolts snapped off in the process.
I'd take a close look everywhere two pieces of metal come together and see just how bad it actually is.