Does the voltage go up when its revved a little? If it does try turning the idle up a little or get a smaller pulley for the alternator. Unless your running electric fans,fuel pumps and water pump. That 63a will be just fine. I have all the above plus a little and only run an 80 amp with no issues. You could put a 150 amp on it and its still only going to put out only 14.5 +/- volts max. You have either or, an rpm problem, poor connection, bad battery, faulty alternator maybe even a bad gauge, make sure your chassis ground is 100% solid.
