This video shows one method of removing bolts that are broken below the surface. The example is done on an aluminum cylinder head, but works for cast iron as well.
PLEASE NOTE: There are some health side effects of breathing Zinc Oxide fumes, so be careful, at least grind away any galvanized coating before welding to minimize the fumes. For more info, please follow this link: https://www.osha.gov/doc/outreachtraining/htmlfiles/weldhlth.html