Even with good gas protection and proper speed/heat receiving good colour when MIG welding thin stainless is fairly really hard to perform. What will be the joint setup like? That is an AeroTurbine 4040xl muffler. The products information states "composed of top of the range 304 stainless steel". Alloy 409 https://www.madanijayateknik.com/service/detail/as-round-bar-plate-sheet-stainless-steel-409-409l-409m-409cb-409ti