IMO, a long rectangular tube NOT welded to the floorpan is a lot more likely to twist torsionally than one that is. As far as the Cartool contoured types, even though they use the floorpan as the "top" of the box, the fact that the floorpan is not flat, but instead has vertical dimension changes adds to their resistance to torsional twist, and reduces shear stress on the floorpans.
Grant