The 8600 is one of the easiest cases to open. With the side panel off, which you figured out already, there two green latches inside. Flip those up and you'll be able to rock the part of the case containing the power supply and drives up to reveal the insides of the case. It's very neat how it's done.

Run your mouse over the case in this movie to show how it opens.

http://www.everymac.com/systems/apple/powermac/media/easytower_movie.html