Use a minimum of .120 wall tubing. Thats just shy of 1/8"
I used 3x3 for the connectors in my FrankenDuster. I made a template of the floorpan out of cardboard and traced it onto the tubing, then cut them. Mine were welded to the front and rear frame rails AND to the floor. I spread seam sealer on all the edges, then sprayed rubberized undercoating on the entire floorpan. It looks as if it left the factory that way.