I haven't cared enough to upgrade the speakers with brand new aftermarkets, so this was a good find. I didn't know 1G CL's had premium sound so I was kind of excited when I found these.
On the underside of the speaker cover there is a little elliptical wall of plastic (echoes the speaker shape) that protrudes downward (so perpendicular to rear deck surface). The OEM speaker just barely fits inside of it. The Bose speaker is a little bigger, so you have to trim down the plastic in a couple places so it clears.
The metal bracket the speaker sits in mounts to the metal part of the rear deck, and the two fasten together with four 8mm screws. I cut up some leftover garage door molding and glued strips around the circumference of the metal bracket. This puts some insulation between the speaker and the car, which results in cleaner sound. Especially when it comes to the bass.
When I go back to the yard I may see if the fronts are 6.5's and if they will swap over too.
EDIT: Here is a pic for you Richard, showing what I did with the molding. I cut strips with slits in them to make them hold the curve better, then glued them on.