I did the same sort of repair you are looking at a few years ago with JB weld. I followed all the good ideas the others are naming here, drilling, wire brushing, veeing the crack, preheating, someone even mentioned hooking up a shop vac to the drained radiator during the repair so the goo gets sucked into the crack. The repair worked great, but after sitting through a winter I noticed a bit of seepage again, which I still have.
But I just keep driving the old 216, checking the water regularly. Sooner or later I expect to drop in another motor, but in the meantime I'll keep running with a little colant leakage. Why not, every other engine fluid leaks too... but the dang thing runs great!
