I am back halfing a dart and was looking at sw racecars for frame rails since they are close. They only have .080 wall 2x3 . Is this thick enough? I thought most people used .120 ?

If you use what they sell the way they want you too, it should be just right. I went thicker, but wish I hadn't due to added weight.

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

I use .083 but I make my bars go thru the 2x3 and weld on both sides

Thanks for the advice. S&w is close and if i get the rear frame and roll cage from them it really saves money on shipping and the frame rails were only $250. I would spend at least 1 day or a little more making them my self. There supposed to be contoured to fit the floor.so i can keep most of the trunk. Probably just cut out the spare tire well.and widen wheel tubs.
Running the tube throuhg and welding sounds like a good idea too, that keeps from tearing the side of tube out.

Unless you are building a streetrod with no cage. Use .083". Anything more is a waste.

Thats all i use...from s&w.