There is a small spring loaded tab near the end of the lock cylinder that holds it in place. There should be a narrow slot in the steering column housing where you can stick a small screwdriver or other tool in to push & release that tab.
There is a photo here -->
http://www.justanswer.com/classic-ca...0-cutlass.html Scroll down to the bottom. Sometimes there is a thin bit of metal casting "flash" covering the slot ... just poke the tool through.
I think it was around 1979 that GM changed over to the screw that retains the lock cylinder.