I JB welded a crack in the plastic top the radiator of my hyundai many years ago. It held the whole 5 years I had the car. It was right on top in the flat part.
I have tried to do the same thing in other cracks in radiators, but they were in corners and bends and other areas that didn't hold well.
Getting it to hold well is all about the prep work. Partially drain the rad, rough up the surface, clean with rubbing alcohol, mix the 2 parts exactly 50/50, let it cure for a full 15 hours like the instructions say, and longer in the cold. Use a hairdryer for a few min after it sets up to help it cure better. Shit, give it a shot and see how it does.