What you dont see in those pics, but I think CRD has one in the link, we used 22 ga to make chanels to support the floor sheet and make body mount points. Bent it up in a 1X2X1 U with 1" flanges to weld to.
The floor & trans tunnel sheet is all 16ga. We dont have a brake (Yet, thats on the list of DIY projects next) so the big bends we did were angle iron in a vise. took a cut off wheel and scored the bend on the inside enough to make it easy to bend with a sharp edge, then just welded the cut/inside bend back up once it was formed correctly.
the floor is bomb proof. nothing flexes at all, and will be plenty strong. The sections where the seats and seat belt mounts will attach will be reinforced underneeth with some 1/8th flat bar where the mounting points are.