Block crack

1) Put white thread sealer on the head bolt & run it.
2) Add a v-groove to the crack & sealer + #1 above.
3) Use the drill&fill stitch method above.
4) Grind the cracks out, weld them up with low-amp Nickel rod, or spray-weld them. Drill & tap it for an insert/Helicoil.
5) If You try a chemical sealer, bypass the heater core, & make sure the rad is fully clear & flowing.
6) Do nothing, clean it up & put it together. (I'm not advocating this) However, this very situation was present on the Olds 455 project I'm working on, We replaced that block, but for another reason. It was running 12.20's at the time, and never leaked a drop, nor lost a head gasket.

Your choice.