well, i don't know to much about anything and i have never tried to mig weld cast iron, but i don't think it would be my shot at fixing the crack. i have welded cast with cast rod on arc welders and even with preheat it's a gamble. lots of time the crack will extend as it cools or crack off from it. i think the gas welding would be easier to control. why not try to braze it? that is the old way of doing it. the cast doesn't have to be as hot for the brazing with brass filler as it would for any other type. even then, it's not a sure fix. honestly, if you dodn't have some time on gas welding, i would just jb weld it- it's prob the best shot to work unless you want to go to the trouble of the drilling/tapping stich. whatever you do i would try it on some scrap motor cast first like jerry suggests.
