That's the way it should always be done. Tubing landing on the 6x6 plate welded to the floor pan doesn't tie cage into chassis very well
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.
2 things. The top part of that rail (the floor) is only spot welded to the top flange of the rail, and both are formed from much thinner material than floor plates or even .083 box tubing. So now you have piece of .032 sheetmetal with 6, 8 maybe 10 1/4" spotwelds attaching that floor plate to that frame rail. Now there are some builders who will go through a unibody car and stitch weld all those spot welded joints to stiffen the body structure before even starting on the chassis, but usually only on higher end Super Stock, Comp cars, etc. Also, by using connectors or "outriggers" between the framerail and the rocker, which is also a substantial part of the strength of a unibody, and even welding that connector to the floor pan between the framerail and rocker, the body and the cage both benefit from the additional support.
