In this case, I don't see a need to stop drill the end of the crack. The cause of the crack was pressure placed on the metal from freezing. Since that pressure will not be an issue, unless you leave the boat in freezing conditions, the only pressure will be from less than 15 PSI of water pressure, if you use the pressure relief valve. If you use a gate valve you can still pinch off the water to reduce the pressure in the block. I would still go with J B Weld or a similar substance. If you already have the block out, it can be welded. The key here is to preheat the area, before welding. I usually have my torch set up on the weld area, get it bright orange, leave the heat on it, and weld it up. This minimizes the heat affected zone of the weld and is a permanent fix, until the block freezes again.
I agree with Cliff about using JB Weld on an crack in an oil passage, but I have used JB Weld on a high perssure air conditioner line in an old Toyota truck and it was still holding three years later when I sold the truck.
Well, you have my 2 cents, hope everything works, see you on the water.
