Jim,
The only way 6.5" speakers will fit in the doors is with some significant modifications. The opening has to be enlarged a bit, and it does have a metal support ring that will make it tricky. More importantly, there is a steel door support behind the speakers, which contact the magnet on the MB Quart 6.5" component driver. If it isn't notched, then you won't get the speaker deep enough and will have issues closing the doors. I measured it all out and determined that 5 1/4" or 5 1/2" speakers were the largest possible without modifying the door panel and the door support. 6" will fit past the door support, but the opening will need to be trimmed, making it impossible to put the stock speakers back in without an adapter ring. However, I have seen guys with new door panels (save theirs to return car to stock) which house a 8" sub each! They removed the door support, which is not recommended.
The doors are set-up pretty good for carrying bass, and have the perfect spot to mount the crossovers. Some acoustic self stick foam on the inside fiberglass door surface, and the mid bass sounds vibrant.
Also, due to the location of the tweeter in relation to the mid bass on the door, I would suggest you wire the tweeter with reverse polarity (positive to negative). This will induce roughly a 3ms delay in the tweeter sound, allowing the mid bass and highs to reach your ears simultaneously. Leave the rossover at -0 Db (for MB quart Components). I don't suggest using 2 ways or Point Source speakers as they don't handle as much power, and some of the mid-bass is deflected by the tweeter. You wouldn't think it makes a big difference, but it really does in the Viper where the mid bass is critical. Also I wouldn't suggest using the stock amp with the aftermarket high end speakers. The stock amp just doesn't have enough cahonies to drive 'em properly to get a good, crisp sound.
Lastly, PHL makes a good driver in a 6.5" as well. Depends on the music you enjoy. The PHL is cleaner, but doesn't hit well below 44Hz or so.
Rdustman, not sure you will get the bang you are looking for with twin 6's under the dash. The acoustics in the car just aren't very good, and I imagine your dash will rattle like my pappy's teeth on a Sunday cruise.
Remember, this is all my opinion driven by my experience. Good luck. Any questions, let me know.