I've always kept my trucks 6 volt. I just replace whatever battery cables are on the truck with new 2/0 cable, & an extra ground cable from this same 2/0 cable. Then install a 6 volt Optima battery, & away you go. Just about everytime, i've come in contact with a slow turning 6 volt system, somewhere down the line the original large 6 volt battery cable, has been replaced by more modern smaller battery cable.
