It may require a small pry bar, wedge type tool and a hammer, tap the bar gently under it. If and when it starts to move stick something under it to hold it and move the tool over and repeat process.
Take your time, it will come off. Since it's aluminum I would start on the bottom in front right where it starts to bend up to mate to the head. You will want to be careful so you don't take an ear of the head surface. Start from the bottom, the flat area in the front move side to side.
