cracked slant block

If midway between pan rail and freeze plugs, then this crack may goes up at a steep angle into the casting to reach the water jacket, so may you have something like a flat area of iron lifting up. I have seen this type of 'scabby' cracked area in freezing cases. It's harder to properly get back deep into that angled crack and fix it with anything. But if you can follow it up through that crack and bevel out the area, then possibly there is more area for something like JB Weld to successfully grab onto.

Sounds like you are gonna handle it one way or another!