I would use either a '55 gauge or another '57 gauge and a cluster voltage regulator, not a wire wound voltage drop. The wire wound ones have to heat up before they start resisting enough to drop the voltage. The '56 are 12v but can be hard to find since they were 1 year only.
The cluster voltage regulator looks like these and can be had from most parts stores. Get one that has the spade terminals, not the snap terminals.
https://www.google.com/search?q=ford...w=1920&bih=937
The T-bird suppliers may be a good bet but keep in mind that the T-bird tank depth may be substantially different from the wagon tank. I would look for a sender that has an adjustable length float arm.