Having just completed work on the steering column of a '64, and reading several of the older threads on the topic, what you'll really need to focus on is the shift lever itself. The detent plate is much easier to replace (if the roll pin on the shift lever is as stuck as mine), but is also made of a much harder metal, so I doubt you'll find any wear on the old one. The shift lever will have grooves worn in it where it has been in contact with the detent plate...that's what's allowing it to slip between gear positions too easily.
