Do you mean tubes as in round? Or box section type stuff?
I used 3/16th box section 2"x6".
I made mine to long though, for some reason we wanted it to line up with the edge of the fender flares so there isn't much room for a larger tire. Someday I might have to take the ends off them and then that might be the end of them being air tanks, they were a bitch to weld up air tight so I can't imagine it happening after cutting off a couple inches and trying to weld on caps while laying down under them.
As for putting them in we cut out past the pinch seam, right up to where the floor started, we also moved the mounts for the seatbelt from inside the rockers to inside the XJ and welded them to the 2x6 once it was in. That way everything was flat and it was easy to put the rockers in.
I have some pics but they are on undeveloped film somewhere.