Use an inline resistor for the heater fan motor. The same for any origional dash guages. Check to see if the fuel guage can take 12 volts, if not use the same inline resistor.
Change the bulbs to 12 volt.
The 6v starter should work fine on 12v. No loss if it takes a few hundred starts to burn out. We use 6v ford starters on 12v all the time. Just don't run them real long. If it burns you should have a rewinder close by. Normally cheaper than off the shelf for hard to find parts.
Its a common thing to have a second battery in a car just for starting it. If thats what your worried about. Normally you can get quite a few starts out of one before you notice any change. Then just put the old charger on it over night and your good to go. Starts 12v but runs 6v