Do yourself a favor and upgrade the charge line from the alternator to whereve it hits the battery or solenoid, both sides of the ammeter... that will make it safe for the extra capacity.
If yuor gauge is a center -0- gauge, then it is an ammeter... if it is a 0+ gauge then it is an amperage meter. What difference does it make? An ammeter is generally used to show net charge/discharge to the battery.
Ie.. if you have a 63a alternator, and have a 40a draw for electrical load, and the battery is charged up.. your ammeter will set just above -0-.. even though that charge wire is carrying about 41-43 amps.
An amperage gauge would show total output of the alternator. Just telling you this so that you don't have some false sense of security that since you don't see high numbers on the gauge that you 'think' there is not high current being carried on those wires.
Thus my recomendation to upgrade the wires to at least 10ga, or parallel in a 10GA wire to/thru the harness, and from the ammeter.
If your ammeter is a discreet gauge... swap it out for onbe with a larger range. ( most ammeter are shunt-voltmeters.. occasional out-of-range usage is not terribly detrimental to them... )
soundguy