From memory Ford use a bimetal arm and contact points in their gauges .. Fuel Oil Temp. The Ammeter measures Amps, not Volts, it doesn't need changing , it will work fine on either 6 or 12 V systems .
With the points closed current would heat up the bimetal arm , it would then bend and alter the points gap thus affecting current flow . There are points at each end ie the sender unit and the dash gauge itself .
Fitting a 6 Volt regulator to power the instruments is cheap and simple .... most electronic shops will sell one for less than 5 bucks , you need to mount them on a heat sink and use caps to filter any spikes . The TO3 case can handle 5 amps I think ...
http://www.ford-trucks.com/article/article.php?id=040
This is a well written article ... step by step . Print it out and take it to any electronics shop to buy the components .. solder it together and away you go ...cheap as chips
MIKE