JB Weld is good stuff, but I don't know it I'd repair a radiator with it, I personally would have it repaired professionally, if your handy, you could do a solder repair yourself...................Tom.
Take it to the local radiator shop. They should be able to solder it shut, and pressure test it for under $50. Or as stated above, if your a decent handy man, you should be able to tackle it your self. Clean it up good and solder it with a small propane torch....just be careful that you don't get it too hot and cause a leak somewhere else!