Should not be a problem. I've done this a few times and have never had any difficulty with switches. Gauges, motors, radios...things like that will be sensitive to 12v, though. Your fuel gauge should not care as long as the voltage to it is reduced to 6v.
A relay for the headlights isn't a bad idea at all, but generally I've not found it to be necessary. However, in honesty a relay will keep your headlight switch from wearing out as fast. A month or two I showed what had happened to a headlight switch over years of use and how to make it work like new again. Don't forget your dash lights and hi-beam indicator light. Also your interior light above the seat. If they're 6v, they need to be changed to 12v. Good luck, Tim. Sounds like you have a pretty good understanding of this.
