If you type in "seat belts" in the search box, you'll find some other threads discussing the subject.
I'm assuming that you're speaking of the buckle covers with the GM logo. They can be removed, but must be done carefully, unless you're planning to replace all covers. They crack easily, especially something that's 38 years old! If you flip over a buckle, you'll see an area where you can use a slimblade screwdriver or knife blade to pry each side off. Be careful and go slow, and work each side a little at a time, or you'll crack the cover and it'll come off. Inside, the GM logo and spring just pops out. You'll notice that it just fits in the four-cornered cradle when re-assembling it. If you're planning to seperate the belt from the buckle, simply slide the knarled beltstop roll down, and you'll be able to slip out the double-thick stitched belt end out and away from the buckle assembly. If you plan to clean the webbing, use a mild soap with a soft brush to wash them, and don't use any bleach, otherwise it may weaken the webbing. Hope this helps you. I'll attempt to look up some photos.