A necessary step in sheet metal and plastic repair is to drill a relief hole at each end of a crack, to keep the crack from progressing. I've successfully repaired cracks in radiator headers and tanks that way. (I know it goes against the grain to drill a hole in something that already leaks, in order to repair the leak, but the technique works.) I've not tried that technique on cast iron, but since cracks tend to progress with time, I think that method is worth considering, especially since you are considering a cement-type filler. Since you are planning to grind the crack to open it up, grinding a radius at each end would serve the same purpose as a relief hole.
Let us know how it goes.
Harvester
