I don't know what JB is? it must be an American item (uk here) I would use a Cyno thin type glue to put in the split, (clean it as good as you can first) relieve the pressure in rad by taking off the cap and making sure the water level is below the split.
give it 30 mins to glue, then I would strengthen the outside of the rad where the split is, with the mixable Epoxy type glue, I would build up a thickness of this glue around the area of the split, put atleast 4 layes of this glue on, waiting 15 mins after each layer, (if using 5 minute epoxy or 45 mins if using 30 minute epoxy) 30 minute epoxy is the stronger of the two.
bear in mind, glue is a very good fix, but preperation of the area is all dependant on how strong the astiction will be.
give it 30 mins to glue, then I would strengthen the outside of the rad where the split is, with the mixable Epoxy type glue, I would build up a thickness of this glue around the area of the split, put atleast 4 layes of this glue on, waiting 15 mins after each layer, (if using 5 minute epoxy or 45 mins if using 30 minute epoxy) 30 minute epoxy is the stronger of the two.
bear in mind, glue is a very good fix, but preperation of the area is all dependant on how strong the astiction will be.