Judging from your handle I'm guessing this is for a 55 1st series?

The ammeter isn't voltage sensitive, it works on 6 or 12 volts. The temp and oil pressure guages are mechanical and therefore don't care what voltage you're using. That leaves the fuel guage. I know of some on here who have done absolutely nothing to their 6 volt fuel guages and they have worked fine on 12 volts for many years now.. Another option is to get a voltage dropping resistor designed just for this purpose from most any of the vendors that are out there. The only other thing is to change out all the cluster bulbs to 12 volts.