I would just wire it straight to the 12 volt. I did this with the original horn from the '51, and more recently with the accessory matched horns I installed. The horn is like the starter-it doesn't mind 12 volts, just don't hold the button for minutes at a time, or it might fry. Make sure the contacts inside the horn are clean. It should sound the same on 12v as 6v, only LOUDER.

-Tim
'51 Chevy 3104 w/'56 235, 848 Head, HEI Dizzy, Corvette Cam, Split Manifold and Dual Carter YF's