You shouldn't need to take out every screw. I took pictures to help you identify the pieces holding together the back plate.

First, remove the bottom plate and unscrew the board that is attached to the bottom plate.

Next, remove the screws under the tower until you can remove the back plate on the tower as shown.

The green piece in the picture is the Z-axis spacer. You can install that now--make sure hollow section is on the BOTTOM as shown in the picture. It should click on when installed correctly.