cracked radiator boss

Cast iron or steel radiator?

What sort of repair: when you say cracked do you mean there is a small crack which is leaking but the system still looks structurally relatively sound, or do you mean it's in two or more pieces?

I wouldn't bother, it's unlikely to seal and, if it does, the crack will be corroding away and will probably start leaking again. Chuck it out and get another radiator. Don't wind the tail in so hard.

None of the chemical metal things are of much use when it comes to repairing that kind of crack, whether on a radiator or a hot cylinder (I tried that once) as the thermal cycling buggers its adhesion. If it's a steel radiator, I would just weld it up.