One #8 wire will handle both relays since only one at a time will be working, or even a #10 would be plenty. I always mount the relays on the core support close to the lights and run the big power wire out there. That shortens the circuit from the relay to the light. You can use the original headlight wiring from the dimmer switch to trigger the relays. The only reason I use 60 amp relays is the size of the wires in the socket, but I'm always into "overkill"- - - -that's from my dirt track racing days- - - -"Build it three times as strong as you think it needs to be, and try not to be surprised when it breaks!"
Jerry
