[dmjlambert]

Check how much voltage drop you get with accessories running at both sides of the firewall bulkhead connector. You probably have a 10 or 12 gauge red wire going into a position on the bulkhead connector. Measure it there by inserting your test meter probe into the back of the connector to make contact with the terminal. This is done while the bulkhead connector is connected to the firewall. Inside the cab repeat measuring at the connector. There's not anything to tearing apart the fuse panel, no biggie. It just fastens to the firewall with 2 screws and you move it out of the way to gain access to the firewall connector. While you have it unfastened, you can make measurements of voltage at the terminal connections on the back side of the fuse panel. I think you'll be able to figure out the main cause of the voltage drop. It's probably one of the connectors if I had to guess. It might need a cleanup of the connector.

I've been rolling it around in my head about how best to get the main power wire voltage into the cab without relying on just one position of the bulkhead connector, to improve the wiring in my truck. I haven't settled on any plan yet.

With all that said, the voltage drop is probably not a serious concern unless it's actually causing you problems. If it's just causing you worry, that may not be enough. These trucks were not engineered to have stellar electrical systems. So, you could power the gauge from the original black/white wire previously going from the alternator to the ammeter you replaced, or power the gauge from the original black/brown wire previously going from the battery to the ammeter you replaced. One of those wires will probably give you a better and more steady gauge reading.