A universal gauge with the preferred markings can be fitted into the stock dash with a bit of effort and some ingenuity (I’ve done it before). Simply buy a typical round 2” gauge with the markings you prefer. Remove the bezel by twisting the edges with a screwdriver (which destroys the bezel), remove the bezel and the glass, unscrew the nuts on the back of the housing and lift the gauge unit free. Mount the “guts” of the gauge to your original panel fabricating whatever adapter is needed to make the gauge face line up in the window correctly. This works for just about any type electrically operated gauge.
The following photo shows the temperature gauge in my 46 Ford. There were 3 more gauges to the left which aren’t shown in the picture. The fit wasn't ideal but it worked great. And using the same technique, the speedometer and clock were also transplanted from a Thunderbird.
The problem is that ammeters are obsolete. Unless you are restoring the truck to pure stock it should be replaced with a voltmeter. Here’s a typical warning, this one from the Ron Francis catalog (their web tech tip link doesn’t work):
“DO NOT USE AN AMMETER! WHY? Ammeters risk under-dash fires. It probably reads 30-0-30 or 50-0-50? What amperage is your alternator? If it is more than the ammeter, say 100 amps or 75 amps – you can understand how a low amperage gauge can’t handle what the charging system can put out. You must not exceed the capacity of the gauge!”
Essentially an ammeter requires heavy gauge wire for all the vehicle voltage to run through the gauge, and gauge failure can burn up the vehicle. This is more likely if the electrical system is upgraded with a modern high output alternator, since the amperage will probably exceed the capacity of the factory amp gauge.
Voltmeters are connected more like a light bulb. Here’s a link showing typical installation instructions, should you choose to go that way:
http://www.ehow.com/how_4840382_inst...olt-gauge.html