Is the radiator mounted in rubber grommets? Try running a ground strap from radiator to eng block, see what happens to voltage. If you can't get rid of all the voltage, you might try a
zinc anode in the radiator a-la marine engine cooling systems. They are available to screw into a threaded boss in the engine or heat exchangers(radiator) If you don't have an extra boss on the radiator, replace the drain plug with the anode. The anode won't get rid of the stray current, but it sacrifices itself to save the other metals in contact with the water, and it must be replaced periodically.