As with most stuff like this it all dcepends on how good your welding skills are, anything is possible its just getting access to weld is the problem, if it were me I would cut out the entire section of rust in the floor including the chassis/body support (in one piece) neatly and squarely then weld the new chassis/body support to the new piece of floor, mount this back to the chassis, and then weld back to the floor, this will limit the amount of welding to be done upside down which is a pain in the ass. You will however have to seam weld the chassis/body support back to itself aswell (upside down). Good luck