One way I use to tell if an alternator is going bad is to get the engine running and take a screwdriver and put it near the center of the rear of the alternator. If there is significant magnetic pull, it's usually good. But keep in mind, it's shade tree mechanics and not 100% accurate. One sure fire way to tell is take it to an auto parts store and have them bench test it. You can always upgrade but if you don't have a lot of accessories needing extra power, why spend the extra money?

1959 3100 Apache Fleetside