My experience with mig welding has been very good. No special preheating or slow cool down used. The only failures were in an area that was spider webbed and the many tiny cracks filled with carbon from the exhaust. This usually was under the metal shield used to duct heat up to the air filter housing and it wouldn't take brass or nickel either. On clean grey iron, like welding a flange on for a split manifold, just strike an arc and weld away--no short beads and cooling necessary and just plain old wire. Cut a junker in half, vee the ends to grey iron, and practice; I think you'll be pleased.
