what do you plan on doing with it? off-road needs to be a bit thicker than street/track. if adding suspension, significantly thicker.
monocoque, semi-monocoque
DCC - go to my thread "another Kart project finished" You'll
see I used thinwall 3/4 inch OD tubing - I can stand (over 200lbs) on the side rail and it has next to no flex. You would have to have a very large single tube to be able to do that. It's lighter AND stronger than a single bar frame. It does involve more welding and cutting though.