Your 10 Ga wire is fine. Remember that the higher the voltage, the smaller the wire has to be to carry the same power, therefore the same wire can carry twice as much when you go from 6v to 12v.
Your problem is going to be the ammeter. You can't put that kind of current through a 30 amp meter. I always like to change to a voltmeter and remove the high current wires from inside the cab completely(single wire alt run directly to the starter post).
But if you don't want to change things, you have to connect a shunt across the ammeter so that 2/3 of the current bypasses the meter. It will still indicate if the system is charging, but the numbers won't be right.
