I would have drilled a small hole through the floor where it covers the top of the frame and gotten to it that way. I know people who have drilled those weldnuts out and tapped up to at least a 1/2-13 (though that's pushing it) - they aren't all that small, plenty of meat in there to work with.
Going through the side of the frame I'm not sure how I would have fixed it up after - the problem being corrosion resistance after the fact, of course. Expanding foam is cool except the risk is that the inside wall of the frame isn't perfectly clean and will wick water into the bare cut metal and allow it to corrode, welding a patch over it is cool except that it burns all the protection off the inside and then you can't protect it at all. I can't really think of a good answer right now, aside from boring through the floor and getting at the nut that way, or boring the nut out and tapping to a larger size (preferably loading it with antiseize at the same time!)
I'm still intrigued by how jeeps rust differently in different areas of the country - up here, things rot out and leak to the point that my crossmember nuts and bolts came off with not a single problem, since they were soaked with 20 years of leakage from the front diff, PS system, oil sump, and trans cooler lines all rotting and/or seals being aged by weather and salt. I just put a wrench on them and unscrewed them.
But you could fit a shoebox through the hole in the passenger floorboard, and there was almost nothing left of the bottom of the frame rail behind the crossmember for about 3 feet due to corrosion.
And somehow you guys get beautiful sheetmetal, but your fasteners are impossible to get loose.
