How do you remove the armrest?

Most if not all have phillilps head screws that go through the bottom of the arm rest and screw into the door panel.

Most I know, but not the impala.

While I've not personally done a 58, every Chevy arm rest I've done has been the same setup. The only difference is the spacing due to the size of the arm rest.

Just a wild guess for you! If you open the door, then get down on your knees and look at the underside of the armrest do you see 2 holes in the underside of the armrest? If so, get a penlight and shine it into each hole - you should see the head of a Phillips screw way down deep in each hole.

There maybe a smoothing plug hiding the hole, these often have a small hole to pry it out and uncovers the screw holes.
Tony

Thanks guys. I'll check it out.