I've done a JB weld repair on a 3 - 4 inch crack on the block (in place), and it went just fine and is holding up great. Sorry, no worthwhile pictures. As mentioned above, I drilled out the ends, vee-ed the crack, and really cleaned the surrounding area well. I followed a few other suggestions that the friendly stovebolters came up with. Of course the coolant was drained, and before applying the JB weld I preheated the crack, and I even attached the shopvac to the radiator so the JB weld was sucked into the crack.
