The ARB instructions you attached from Quadratec states each of the 2 red (+) wires is 10awg and the main black (-) wire is 8awg. ARB says if a splice is made the wire should be the same gauge or larger.
Google "Winchester 1897 riot shotgun" or "Winchester 1897 trench shotgun".
I have not seen the movie but this may be similar to what you are describing.
Kolpin ATV Fuel and Water Packs
Kolpin ATV Fuel and Water Packs at Cabelas and Walmart
http://www.cabelas.com/cabelas/en/templates/links/link.jsp?type=pod&id=0049067
