Where are you looking to purchase a radiator from? Could you pay extra to have them turn the fill neck? I am assuming you are looking for a replacement brass/copper radiator since you are concerned about the position of the overflow outlet, so your choices are probably pretty limited.

That said, I know the Champion aluminum radiator main outlet will modify one of their existing radiators to fit your needs. I've also seen a few racing radiator outlets that will build the radiator you need, but you have to get the radiator from the people that build them, not one of the retail outlets.