I just finished a backhalf on my Dart i made my own rails using rect. mild steel 2x3x.083 as i pieced it together id physically try flexing the rails and as i added bars such as the down bars it would stiffen but i could still see some flex so i added X bars and a few extra lightweight tubes to get it stiffened up, so by the time it was finished i shoulda just used .120 or .095, i also had a heavy friend who i had to grab the rails and try moving them so i could see where the flexing was occurring, its very stiff but as i said next time id look for somthing thicker than .083 mild steel on a frame structure