Success! At first, I tried cutting through the rubber bushing and the bolt with a reciprocating saw, but I was creating more burnt rubber smoke than making progress through the bolt. Plan B was to use my bi-metal hole saw to cut a window through the passenger side floor, so I could figure out another course of action. Once done, I used my air chisel to remove the sheetmetal housing that surrounded the nut. I then welded the nut to immobilize it. I put my breaker bar with the 2 foot cheater pipe to work, and instead of the bolt finally coming loose, it snapped. It failed right where I had gone about 1/4 of the way through it with the reciprocating saw.
The hole doesn't matter to me, because I had already planned on replacing both front pans anyway. As for the partial bolt and nut, I'll just cut that area out and rebuild it with new piece of sheetmetal, and a large washer with a nut welded to it.