well if you want to use your welder i would go with 1/2 inch for the main section and then 3/8 on the cage....but i would go with 16 gauge steel tubing.....the brake line is too thin to weld you can braze it but i wouldnt be as afective as the mig......
hers a drawing i did for one of my future builds.
and i used 1/2 inch steel conduit from homedepot and it works great i think i went over board on it...lol b/c i weigh at 297 and it holds me with no prob at all....
heres a pic of it it stands where it is at right now im not done just yet.!
its a 1/4 scale petrol rock crawler.