Max current rating for (low voltage) 10 gauge is 55 amps (60 deg C, free air)... so at minimum, protect the charging circuit with a fusible link sized for the 10 gauge wire. That would be a #14 fusible link.
Or, to fully support the 63 amp alternator with some margin, consider replacing the #10 wire with #8 for the charging circuitry - wire to ammeter from the battery terminal on the solenoid, ammeter to V reg (if it still has an external regulator) and to the output of the alternator. # 8 wire has a current rating of 73 amps (60 deg C, free air); protect with a #12 fusible link.
Fusible links are selected by gauge size of the wire they are intended to protect. The guideline is to select a fusible link that is 4 gauge sizes smaller than the wire, generally 3-4" in length. Here is a good review:
Fusible Link FAQs.
The current ratings of various wire gauges can be found in the Handbook of Electronic Tables and Formulas for American Wire Gauge; here is a convenient link for the
AWG Current Limit Table.
