I have 2/0 battery cables in my truck. I was able to make them to fit with some battery cable we had laying around at the shop. (Had a box of take-offs from some battery package upgrades on some of our equipment)
My truck cranks over so well that it apparently prompted some debate at a WPC Club meet a few years ago. We were doing a group tour, with several vehicles, of Civil War battle grounds near Chattanooga, TN. There were several stops along the tour. At one stop, later in the day, a couple other guys approached me to settle a bet. One of them was convinced that I had upgraded to 12 volts based on how well my truck cranked over when I would start it. The other believed it was still 6 volt. I assured them it was still 6 volt, but had very good battery cables with clean connections.