I've done exactly the same block repair last winter, and learned a lot about JB weld. The second time I did it, it worked great. I wrote up at length what I did and posted it, but I can't find my own post using the search, so here is the short version.
Drain the coolant and make sure it is really dry around the crack. That area of the block drains poorly, make sure the drain spigot at the back of the block is open and free. You can even try using low air pressure through the radiator to blow the crack dry, really dry. Clean and gouge (vee- out) the crack and keep in mind it could be longer than it seems. The experts recommend drilling a small hole at each end of the crack to keep it from spreading. Try to warm the block before making the repair, I used a hot air gun. Then I attached a vacuum cleaner to the radiator so the crack was sucking in air as I applied the JB weld. I could see the gunk being drawn into the crack. My patch is holding great! If you might have cut corners on the prep work, don't give up - try again. The stuff should work.
Karl
