Power steering pump replacement questions

That should be a bolt-on pulley. Maybe some rust or burr is keeping it from sliding off. Lube it up good with some WD40 and give it a little pry on both side. There is a keyway and it should slide off once the nut is removed. If you have a proper puller, give it a try too. Once the pulley is off, you can clean up the hole and keyway in the pulley before you push it onto the new pump. Use some grease or antiseize compound on the new shaft.