I've seen guys running 6-volt horns on 12-volts. They are louder. Though it likely shortens their life. I think that the 6-volt positive relay has to be replaced with a 12-volt one.
The series wiring is an interesting technique to ponder - but how do you isolate the mounting so that there is just one shared ground path for the series? Most vehicles of this vintage are grounded to the body - be it positive or negative.
