i was under the impression that there is a nut welded to the body from below. this doesnt make sense. go get a 12mm bolt, which can be found in many other places on your vehicle, and put it back in where it belongs.
and dont EVER USE AN IMPACT on something like that. thats lazy and an awesome way to break all sorts of crap.
Sorry to dig up an old thread, but any idea how long of a bolt you used to go through the floor? I feel like dicking around with sheet metal for this is going to be a huge pain. Much easier to drill a hole through the floor to the bottom of the vehicle.
Actually, now that I'm thinking about it, how is the rust in this area? I see your post was from a few years ago already...
It is worthwhile to pull up the carpet and inspect the other spot-welds around the seat mount crossmember also. The nut for my drivers seat ripped thru, and I also found several cracked spot welds. Its best to fix this right the first time.
I made a patch from some scrap steel and welded a nut to it. I was able to slip it in from the opening in the back/middle area of the crossmember. Temporarily held it in place with a couple sheetmetal screws from the plug weld holes. Put a block of wood underneath, and used a seat bolt to push it up tight against the crossmember for welding. Weld, plug weld, grind, test fit, and paint.