Also, you could install an under-bed gas tank, then remove the behind-seat gas tank and use that area for storage or stereo component install. A subwoofer firing through the seat would work good back there.
If done well, Jim, your under-seat idea looks like it would work pretty good. Some purists will cringe though.
Curt, that's a beautiful color you've chosen for your dash. Looks like Viper Blue to me. What steering column did you use, by the way?
Will, you could either check with Crutchfield (but I kind of don't think they'll make anything for us AD-owners), or possibly look into Classic Industries. Classic Industries sell a more modern stereo that is modified to fit in the dash in the original radio location, but I believe it's a tape deck. For a CD, why not install a CD changer under the seat? I guess you could mount a CD changer in the glovebox, and if done cleverly, you might even still have a little storage space left in the glove box. Also, you know, the back corners of the cab, on either side of the seat, would be a decent location for a pair of speakers in custom-boxes.