Thank you for all the responses. Now that I know the 6 volt wiring is ok for a 12 volt battery, can I also use the 6 volt horn?
The 6 volt horn will work on 12 volts but it won't like it and you will drastically shorten its life. Find a 12 volt horn or a 12 volt to 6 volt voltage reducer.
Or, find a second 6 volt horn and wire them in series. if in good condition they should divide the voltage more or less equally. Just don't ground the horn that's between power source and the second horn. I agree that would be rather unorthodox but it should work.
I've said this countless times, why? I have six volts in my T, and it spins over like a 12 volt, and my horn is probably the loudest on the planet. The key is to rewire the entire car, and make sure you have a good starter switch too, which is where a lot of problems come from. I wasn't happy with the show quality looms, so I had mine custom made locally. The guy even wove the original Ford tags into the looms, all three of them. It cost me three times more to have this done, but to me it was worth it.
Back in the '70's and up, when cars went to 12 volts but still had points & condensers and coils, they found that 12 volts would burn the points and burn out coils if the switch was left on with the engine not running.. So, they used a dropping resistor to have the car run on about 6 volts at the coil. Then they found they were hard to start when cold, so they added a contact to the ignition switch that sent a full 12 volts to the coil when the switch was turned to "start."
All this is to say that if you go to a junk yard and find a car from that era, you'll probably find a dropping resistor they will sell you for a nickel or so, if you remove it yourself.
What you're looking for is a ceramic block about 3/4" wide and 3 or 4 inches long, with a coil of wire visible inside it and a terminal at each end, attached to the firewall.
I can't assure you that the resistance of that thing is 'just right' for your application, since the amount of voltage drop depends on the load, and it was designed for a different load. But given the cost, it would be worth a try. You might try getting two of them while you're there, and play around with putting them in series or parallel if they don't do what you want.