What wires to redo first? 1975 custom

Not sure if in 75 the ammeter is like the earlier models or if it is a remote shunt type. If it is like the earlier ones I would look at the bulkhead connectors. clean, tighten and maybe replace and corroded or burnt terminals and replace any melted plastic.

Do you have any reason to believe the wiring is suspect?

The bulb in the ignition switch is most likely incandescent and will generate heat. it may have been changes over the years to a brighter( hotter) bulb so that would be something to check