273 Radiator for 340?

PS. With a soldered copper-brass radiator and heater core, be conservative and use traditional coolant with inorganic corrossion inhibitor. That's the green stuff, but not the hybrid organic acid technology (H-OAT) that is often sold as "universal". Shop carefully. None of the other stuff has particularly good protection for the solder, and generally less protection for the copper until well established.