Editing my original response, because I realized you were talking about simply replacing the stock radio but keeping the look.
The only place I've seen that done is at the site linked below, and it's a vast amount of work. He basically uses the knobs and push buttons to control an XM radio, if I remember right, and a PC to drive a 2-line LCD screen that replaces the original AM face.
http://55buick.com/Carputer/carputer.htm
I've been tempted to go to extremes like that, but my stock radio works extremely well, so I may trade it to a friend who is hardcore about authenticity and just use the faceplate from his broken radio to fill the hole and "keep the look", then hide an amp and new speakers for an ipod type hookup. (And if I'm feeling ambitious maybe I'll re-use the broken radio knobs to power the amp and control volume, who knows.)
It's interesting that you mention using the old speaker grille as AC vents. That's what I'm planning to do. I want the interior to appear stock when I'm done but there's no way I'll drive a car without AC in Florida!
You know, I've also looked at products like Secret Audio and their "classic" head units, but I don't think they do a very good job of nailing the look, and their stereo equipment is somewhat sub-par and very expensive for what you get. Oh well.
If you decide to tackle this in some way, post up what you do, I'd be very interested. I'll probably start on mine later this coming summer.