I would say yes to welding your cage with mig using an ER70S-2 or-6 wire, but u better be good. We use tig process for less heat input and better and more consistent tie in and that same tig rod either a -2 or -6. They say to use a mild steel rod like what i considered because it gives flex in a crash or roll whereas using a chromoly rod on 4130 would make it stiffer and more likely to snap or fracture in a crash or roll. The only problem i see using mig is insuficient tie in in unpredictable areas,and a broader heat affected zone. NHRA drag cages are all tig welded and stress releaved after welding. If you just doing some dirt track racing at your local tracks you'll be fine. As far as welding 1018 to 4130 i don't know any metallurgical fact on that. I'm sure it will weld to it but as far as strength and reliabilit i have no idea. I don't think NASA would be doing it,but it depends on what your doing or how fast your going to be going. Hope i could help alittle and good luck to ya..