I did a bottom mount in my 300CD and put in a 4" triaxial and used the stock covers. But even with the dash out I had to dremel some metal out of the way.

You might consider these from retrosound. RetroSoundUSA.com - RetroMod&#153 3.5" 30 watt 2 way Upgraded Replacement Speakers (sold as a pair)

The side view shows they stick up a bit, but they may fit a bottom mount at 3.5 inches, assuming you can get a screw in place. There are a few other recent threads about speaker sizes. Crutchfield has some excellent exploded views where you can get a good idea of dimensions.