sqguy/Mike... you forgot several important pieces to you question....
What kind of car?
what kind of budget?
what final results are you expecting?
who will handle the tuning & using what tools?
Mike made some great points about the general install of horns, however horns are VERY vehicle specific. 3 of my cars have them, so YES I am a fan... LOL
But you need to either have the midbasses as much on axis with the horns as possible OR have T/A-Processing available. it helps tuning tremendously. Also, I prefer a "super tweeter" myself, its kinda old school, but I like the extra height & shimmer of a nice soft dome, from about 15K up.
last thing, is the further away from the passengers seating position you can get the hor bodies the best. close in the bottom of your dash (carpet/felt covered fiberboard is fine since its not structural)... think touch the Compression Driver itself as far back & into the corners of the foot well as possible.... helps deepen the sound stage, widen the over all stage presence, & adds a little height.
give us some more details... there are likely people on here with the same car or have done the same car at some point.
Rob