I just pulled the pre-existing base off with pliers or my hands or something, I don't remember it being difficult. The new antenna originally came with different bases for different angles, but I only had the one from my car. There was a rubber collar within a the plastic base, and the metal/chrome screw-on-to-the-antenna piece goes inside the rubber collar, so it allowed me to have an angle on the base. That may not sound clear, but it looks ok. The only thing that might bug someone is the antenna comes out at a slight angle towards the center of the car, it's not perfectly straight up.

cant you shim it?