J-B Weld Company of Sulfur Springs, TX (903-885-7696) told me that the City of Dallas, Texas used their product to repair a cracked Caterpillar engine block. That sure got my attention! They also said that people have been using J-B Weld to fix old cast-iron radiators. But to do this you first have to be able to drain the radiator and remove any paint, primer, or rust from the surface. After cleaning the surface with a non-petroleum-based cleaner (acetone or lacquer thinner), you have to rough up the surface with a file. Then you mix the two parts of the product and apply it to the crack. Finally, you let it dry for at least 15 hours, and see what you've got.
As for temperature and expansion and contraction of a cast-iron radiator, they said that the product actually "softens" when heated and will move with the metal. It's not the sort of 'softening' you'll notice, though. You’d have to get the temperature up to 400 degrees Fahrenheit, to see that (the product is good up to 600 degrees.)
The challenge, of course, is that an antique radiator has a lot of nook and crannies, and a good leak knows where to hide. But if your customer is in love with some old radiator, the product is certainly worth a try. You can buy J-B Weld for about five bucks at most automotive and hardware retail stores.