On the front of the ignition switch you should see a tiny hole next to the keyhole, (on my '62 Chevy P/U its at about the 2:00 position yours is probably the same).If you put the key in and then turn it counter-clockwise, then take a piece of wire like a paper clip and push it into the small hole.Now continue turning the key while pushing the wire in, and the lock mechanism will come right out.

I've done this to a 1960 GMC myself. Its not easy to explain. If you look between the housing and the key chamber, hopefully you'll be able to see two or three pieces of metal bent into a flat u-shape. The trick is to get those out, once you get those there are two paper clip looking things in there. They're even harder to get out than the u-shaped things. There really isn't any trick to it. You'll just have to mess with it until you get a lucky break.