[Hocrest]

sounds like your going to have to cut the heads off of the nuts so you can remove the seat.

Once the seat is out, you can get under the carpet. There are holes in the "beams". I'm not sure if they are located where you'll need them to access that area.

You may have to cut new holes to weld some nuts. If you need to cut, I'd recommend cutting from inside the passenger area so you don't give water and salt a new entry point into the body.