They should be screwed on. There's one on each side, and I think that you'll have to disconnect the dash from the steering column. And maybe pull the wheel.
Here's a breakdown diagram, courtesy of BradB:
http://www.brads41-46chevys.com/id31.html There may be screws in the top, too, but I've not pulled mine, only stuck my head under there to see how it was attached and decided to wait 'till another day to replace my speedometer.
