roll bar - materials???
 

I’m modifying the support arms of a Advan roll bar so that they attach to the rear struts opposed too the rear wheel wells. What material would you suggest I use for the support arms? Should I use 40-50mm chrome moly pipe or could I just use stainless steel or even mild steel? Can I use the MIG welder on chrome moly?

I'd use the same material the cage is made out of. Here in the States, the SCCA requires all supporting tubes be the same size and thickness as the main tubes, so that's something else to consider.

AAACKKK! Find out from the maker what the rollbar is made out of. 4130 CM should not be MIG welded. If you cant find out from them, you can figure out the material once you cut the braces off. Once they're off, the weight per foot and tubing size/thickness will give you an idea.