While you're looking for a replacement block, try smoothing the area for about an inch on each side of the crack with a flap wheel on an angle grinder and apply a layer of Marine-Tex sealer about 1/4" thick. As it's curing, press a piece of 40 grit sandpaper over the surface of the patch to duplicate the texture of the cast iron. As long as you don't use a high pressure radiator cap, the patch should be leak-free. I believe the cooling system on your truck uses a 7 PSI radiator cap, which should be OK. You might decide that after cleaning out the crud in the water jacket, you don't really need to swap engines right away.
Jerry
