Forget it with a mig. Cast iron needs to be welded while preheated and has to stay hot and cool slowly to prevent cracking and some cast irons are not weldable. If all the conditions are not right you got cracks. Mig deposits are made hot and fast and cool too quickly to work. If you can get it to hold that will probably be about it. The weld will fail quickly if placed under stress. I agree that DC stick is probably the best way. Good luck!