I have seen cast iron repaired with small pipe plugs. Clean the area really good, bead blast or sand blast even so you can see the start and end of the crack. Drill and tap a hole half way in to the crack and good metal then screw in the pipe plug. Cut the head off and grind flush without hitting the block. Then drill a second hole part way into the first plug and the crack and install a plug. Do this all the way down the crack. Lots of heads are repaired using this method. Use solid pipe plugs with a square drive protruding, don't use the type that use allen wrenches or square drives that are recessed into the plug.
You can also " V " out the crack and weld it. Preheat the iron around the crack and use the correct wire or rod. Keep the block hot and cool it slowly after welding. Brazing or even silver solder can work. If you are good enough at it, I would bet good old radiator solder would work if you can get it to stick.
Then there is the JB Weld repair. Again " V " out the crack and clean it a couple times, add the JB Weld and let it set up. This would be easier if the block was on its side, the epoxy will flow out smooth and into the crack. If you could plug all the other holes, you can pull a vacuum in the block to draw the epoxy into the crack even more.
The old cooling systems only have a couple pounds of pressure on them or less, so it doesn't take all that much to seal one.
Getting it clean is the hardest part.
Joe
