Just because you replace the wiring doesn't mean that you also have to change the truck to 12 volt.
Seth Daulton and maybe a couple others make full replacement wiring kits for 6 volt.
https://qualityclassicparts.com/prod...pr_seq=uniform
The only problem with using a "12 volt wiring kit" with six volts is that the wire diameter may be smaller than the 6 volt system needs. The smaller wire ends up having a little more resistance that the 6 volt battery wants to push electrons through.
The primary reasons for switching to 12 volts
Brighter lights
install AC or a sound system
12 Volt accessories that you want to add.
If none of those are a must have and you are happy with 6 volts so be it, don't let the local spit and whittle club tell what they think you need to do. Simply meaning that there a number of individuals and especially the Face Book self styled experts who love to tell someone that "Ya gotta do it this way" but have little or no experience.
That brings it to the question of do you actually feel that you need or want to change to a 12Volt system (yes) (no).