Kevs57,
I have read posts that indicate a fuse box on your '57 was dependent on factory supplied electrical options.
It seems like you have the wiring down quite well. The power for the gauge must be switched by the key to turn the gauge off when you shut the truck off...where this happens is up to you.
It could be tied to the key switch, or somewhere more convenient that is switched. This would require a search on your part.
The gauge illumination would, as you state, come off the headlight switch, or a more convenient place that is switched with the dashlights.
You are sort of 'tagging' this gauge into the existing system, so as long as the connection point has the the proper function, it can be where it is most convenient for you to 'crawl' to.
It would probably be prudent to install a small inline fuse in your new wiring to the gauge.
Stuart
