Backing plates would be better welded on the underside.
If you fit buckets, make sure the mounting bars have spreader plates welded into the shell, too.
mounting bars spreader plates ? soz a little lost now
Bucket seats don't, normally, attach using the original mounting points. you therefore need to manufacture new mointing points for the seat to attach to. These are usually, if home made, square tubes welded onto the floor of the cabin through which the seats bolt. the spreader plates are fitted to the underside of the car, in the same way as the plates for the eyebolts, to increase the area (spread) that the nut has to bind to the shell of the car. This means that you are not trying to rip a small nut throught he floor but a large (2" square) metal plate which is, of course, much more difficult.
Hope that explains it all a little better.
jon