what you hafta keep in mind is a parts book covers up to the year it was published, and will include some things that became standard practice after the manufacture of the earlier vehicles - as soon as GMC had used 12v for any application, shops would have been "updating" previous models - GM would simply have made adjustments to parts listing to make it easier for parts dept's to sell folks what they were asking for, and be sure it would fit the application
Bill
