If the weld nuts have broken loose and are spinning, you need to employ destructive disassembly.
I don't want any extra holes in the floor, so I always just snap the shock bolts off and drive out the weld nuts with an air chisel. I have always snaked in a bolt on a bit of flexy wire and put a nut on the bottom side.
Rough Country makes a slip in captured nut/flag nut assembly that I will be using next time.