ideally you want to weld this in an oxy-acy setup or tig.You want no gaps in the replacement panel to original.In a perfect situation you would use no filler rod on either processes jsut fuse the metal together.You will probably need to dab in some rod here and there.Dont forget to grind smooth the backside of the weld and file the front.You want the weld/fuse thickness to be no thicker than the steel you are welding, so when you planish the welds you dont have a hard spot or a spot stronger than the other.Hard to get back to its true shape with that.Mig is not the best option you end up using a pulsing method to weld because of instant heat usually.Doesnt offer good penetration and a hard weld to grind smooth.goodluck