That crack is VERY fixable, but the engine will have to come out to do a good job. I'd suggest a product called "Marine Tex"- - - -it's much better than JB Weld, which has a tendency to rust due to the steel dust they use for reinforcing the epoxy. Marine Tex is designed for wet environments. Smooth up the area for 1/2" to 3/4" on both sides of the crack with a flap wheel in a small angle grinder to remove the casting roughness, clean the area thoroughly, and lay the block on its side with the crack facing up be sure no coolant can creep under the patch while it's curing. Just before the patch material cures completely, press a piece of 40 grit sandpaper firmly over the surface to put the casting texture back into place. The patched area will virtually disappear once the block is painted. Older cooling systems such as yours run a "zero pressure" radiator, so there won't be any worry about internal pressure pushing coolant past the patch. All you'll be fighting is gravity and heat expansion. Good luck! I've got a good 228 engine in the core pile if you are willing to make a road trip to south central Tennessee! 248, 256, 270, and 302 GMC engines would be a virtual bolt in swap if you're willing to go with a "non-original" engine.
Another rust resistant patch material is Devcon Epoxy Aluninum, but I'd reccomend Marine Tex as the first choice. If there's a marina or a boat repair shop nearby, they might even have some on the shelf. Otherwise, it's all over the internet.
Good luck!
Jerry
