JB weld was pretty much made for fixing cracked blocks, that was one of the original advertised applications. Supposedly its coefficient of expansion is pretty close to cast iron.
I used it to fix a cracked dishwasher and it outlasted the pump and motor.
I would drain the block and lightly heat the crack with a propane torch, then let it cool. That will dry out the crack. Then v-groove it, then JB it. I bet it will last a long time.
