As I said previously Warp, that's perzackly the same place mine cracked. I blamed it on a bodgie tie-down on a ferry, with a strap across the radiator. Maybe it wasn't that at all???
I bogged it up with that two-pack Selleys epoxy stuff, worked for a couple of years, but then started leaking again and I gave up & bought a new radiator. Nobody in Oz could repair the alloy tank, said it was too thin to work with.