Bobert is right, ford changed their ohms resistance in the eighties I believe, up to the late seventies all ford gauges were 6 volt so if its a voltage change you are dealing with , then consider all fords from 56 - the late seventies have a volt drop on the back of their instrument cluster, you should be able to find a derilect ford somewhere that you could rod one from.
the volt droplooks like a 12 circuit breaker, the earlier ones have two threaded studs, the later ones have two spades for a pushon connecter
STOP. The original 49' Mercury sending unit is a King Seeley thermal points actuated type. It looks like a little flying saucer. First and foremost, you need to reduce the voltage to your gauges. Some people here on the HAMB use a points style voltage regulator that King Seeley (Ford) switched to in 57'. In our shop we build a solid state voltage reducer based on the application. The points style voltage reducers can fail in a manner that delivers full voltage, thus burning out the gauges. Once the voltage is properly reduced, you can use "modern type" 78-10 ohm sending units. We find that the gauges still need to be calibrated as it is not a perfect transition, nor were they calibrated to our standard off the factory line.
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