Paul,
The GTD (assuming you are asking about your type of car) has seat tracks mounted to steel straps that are welded across the underside of the square-tube frame. While that is about the lowest point of the car, that's often not enough. On my car I removed the tracks and, after installing .125" plate aluminum pans in my floors (sheet fiberglass and a thin aluminum skin doesn't stop too much that kicks up at speed), I mounted my seats directly to the floor. No easy adjustments, but another inch of headroom!
Mark