Just in case you need a new gauge, you can look on eBay for a “Vintage AMPERES” gauge.
There are at least a dozen currently (no pun) available on eBay, although the faces on some are WHITE.
I believe that you can always remove the faceplate with a very ting screwdriver, and replace it with the Black one from your Pierce-Arrow.
The Packard, Amperes / Oil Pressure gauge set is near identical in shape, size and configuration, perhaps, save for color, as that on the Pierce-Arrow.
Some have Black faces.
It will be good for you to start collecting replacement parts, especially electrical parts, for your Series 80, as they are increasingly difficult to find and you should snag them when you see them.
Just don’t tell your wife!
Apropos of the wiring on the car, it will not be that difficult.
You need to buy a number of different colored rolls of electrical wire.
Get one in Black and one in Red, and then a few that are multi-colored rolls which come with White or Black tracers.
I recall that the most difficult wiring is behind the dashboard, as you may have to be lying on your back on the floor and working upward.
The headlight and taillight run is relatively simple, although you need to run it through the conduit that runs fore to aft.
I don’t totally recall, but I may have had to run a nylon string, or just SS-wire through the conduit first and then affix the electrical wire to the nylon string or SS-wire and carefully pull it through.
At least in that way, you know that all of your wiring is new.
The one wire that I did not replace was the wire that runs to the interiorDome Light, as it may require removing some of the “B-Pillar” upholstery to do it.
I rarely use that light anyway.
Restoration Supply and other vendors carry that type wire. https://restorationstuff.com
As for the gauge, I do not recall (a mind is a terrible thing to lose!), however I believe for MOST applications will require 12- to 14-gauge wire.
For some of the direct to Battery leads may require 16 t0 18-gauge wire.
If I am wrong, I am confident that someone here will correct me.
If you ave any questions on any of this, please feel free to telephone.