You have to be careful with GMC parts. Many people don't know what they are selling. Many people use the letters GMC to just mean the General Motors Corporation which includes Chevy and many other vehicles. They do not mean it is for a GMC brand truck. Many parts, however, are the same. The one switch has "C" designations which I thought was only Chevy and says it's for BIG trucks. I would guess that an ignition switch is the same for all size trucks. 12V is 12V. So anyway, I don't see a definite answer from the guru's here yet as to IF there is a difference in Chevy Truck and GMC Truck switch housings. I don't know if you need just the housing or just the cylinder or both. Pictures from a seller on Ebay are nice reference but info from the experts here (not me) is the coupe de grass (whatever that means). And, of course, if you have an old switch to compare, that's great. All you care about is: does it fit the hole and anti rotation feature, and does the proper bezel nut fit. The electrical hookup is not a problem. Don't get too wound up in small details regarding an ignition switch unless you are building a Barrett-Jackson truck. If it's just a Con-Coors de la Glance, it just needs to start.
This indicates they are the same housing:
http://www.bobschevytrucks.com/1963-...art-dr142.html