Actually, The floor on most unibody cars makes up the top half ( welded to ) the front and rear frame sections on a unibody car, which makes the frame a square tubing as it attaches to the unibody. It is just as strong as any full frame car in those areas. As long as your 6X6 plate is welded to the floor above the square tubing frame area under the unibody, you are essentially welding your plate to the frame sections, or top half of the square tubing. Adding subframe connectors, will help stiffen a unibody car, but does not qualify ( NHRA rule book wording ) a unibody car as a full frame car. Attached pics is 6X6 plates welded directly over the rear frame section ( square tubing ) for the hoop and rear down bars.