Tim, I have always used 2/0 cable on 6 volt with great success.. I bought them at a heavy truck repair center. They were able to cut the cable to any length and had about a 4 foot crimping tool to install the battery terminals and ring terminals. Those big cables will spin the starter quite nicely..
I used them in conjunction with a battery cut off switch in a discreet location on both my truck and Kook's. It is nice to be able to kill ALL the power to the truck when it is unattended.
