Being that you are weighing the option of going to 12 volts means you have already decided not to go 100% original.
The companies who provide wiring kits focus on the largest part of the market so they can sell more product and make more profit (free enterprise). That market pie is of people/shops who take classic/antique vehicles and turn them into cruisers, street rods and such. If you purchase one of these kits it will most likely have wiring for a modern GM steering column, 12 volt alternator, 12 volt dash gauges, V8 engine sensors and other modern features.
There are a few companies who make original kits with cloth wiring but I would advise against it because of the expense and it is only logical to use them on 100% original vehicles.
Since you are good at wiring, I would buy the "good grade" professional parts I needed and make my own wiring harness. Try as hard as you can to remove the old wiring intact. You use this as a template. The old scheme was to use the body and frame for a ground path (especially lights and lamps) but I prefer a separate ground wire so I don't have the common old truck grounding problems.
Gauges:
54 GMC gauges are different from a 54 Chevrolet truck. A completely different dash. I don't know why websites show them to be the same.
The new Jim Carter website can help.
https://oldchevytrucks.com/catalogsearch/result/?q=1954+gmc+gaugesRefacing kit
https://oldchevytrucks.com/sp105.htmlI avoid adding-in voltage regulators and dropping resistors to make a 6 volt device operate on 12 volts because of the energy waste. I would purchase a 12 volt fuel gauge and associated tank fuel level sensor with the resistance to satisfy the gauge. Oil pressure (copper tube) and water temperature (gas filled tube) gauges are not electrical on Chevrolet BUT I have no no documentation to advise me what is used on GMC. The ammeter will work with a 12 volt alternator. All the dash lights are changed over to equivalent 12 volt replacement.
