You try to remove the bolt and reuse the nut if by all means you can. The nut is welded to the floor. If you can't reuse it you have toweld a new nut. Remove the old nut and try to get some metal to fit in there with a hole so act as a supersized washer and weld that in then the new nut on that.

The one I helped with we just cut a flap out over the nut big enough to get to it. Folded the flap back long enough to do what we had to then folded it back flat. Once the carpet was put back you couldn't tell.

 

When I did the body lift on mine nothing went right. The bolts run through the mounting point on the frame into a caged nut assembly in the body. This assembly is in a void between the floor and a 2-3 inch wide "rib" which runs across the vehicle side to side. On my truck 5 of the 8 mounts had the caged nut so corroded that it simply snapped the metal which formed the cage and would spin freely. To gt it out we had to cut a hole in the floor and get vice grips on to the caged nut. Some snapped which was good. We simply pulled the old nuts out and replaced them. If we were able to work the bolt out then I re-used the factory nut in that location. To rebuild the ones which snapped I put a nut and washer on the bolt. Welded the washer to the floor, then welded the nut to the washer. You are saying your bolts are super rusty? if they snap or spin free then this is what your looking at to fix it.