I use "0" gauge welding cable. It has a very tough insulating cover, but I still use rubber hose at any point it passes through or near anything that may chaff it. I also solder the connectors onto the cable and double heat shrink those ends.
I also use a full length "0" gauge ground to directly to the motor and to the car chassis. The positive lead is run to a distribution point at the front of the car, where it branches off to various sub-circuits.
