You are correct on the higher amperage rating.
The only concern (if you want to call it that) on using 6 volt wiring loom in a 12 volt system, is the loom is larger do to the larger gauge and weighs a slight bit more and more copper means a slightly higher cost.
All of this is minimal impact to the truck.
The benefits of 6 volt loom in a 12 volt system is less resistance to the current flow so lees voltage drop. Putting a 12v loom in a 6volt truck could lead to failure.
Electrical/Electronics developers prefer the larger gauge if space, cost and weight are not a consideration.
