If you have the original wiring harness in the truck I believe there is only one way you can easily hook it up. One ring connector is larger than the other and the studs on the back of the meter are two different sizes. At least that's the way it was on my '49. The larger one also has a slightly larger nut on it.
Like Bill said, you won't hurt it by hooking it up backwards to test it. The meter reads both directions from straight up, depending on which way the current is flowing.