You need to give more information, is this a water jacket repair, an engine mount, etc.

use cast iron rods take some money with you they arent cheap

Apply MAGNA 770 cast iron welding electrode.
1) Preparation of cast iron befor welding (gouge rod) chip,file, clean wire brush area to be welded, but do not machine grind.
2) open a general area on a 30 to 60 degree angle along crack and drill holes (5mm) at both ends of crack.
3) Select largest possible diameter electrode for crack length vs. thickness of material.
4) Without preheating weaving, run root pass with lowest possible amperes.
5) Clean each pass, peening is only necessary-wire bush between passes.
6) Continue to run stringer beads until desired height is reached.
THE REPAIR IS FINISHED

Do a search on welding cast iron.  There is tons of good information on how to to it.  There are actually quiet a few different way to do it you will come to find.