Speaker placement is always the first,and most important part of designing a great sounding system. If you want to (or have to because of time/financial requirements) you can use factory locations. But even if you only care about the driver's seat listening location, I'd bet you can make a significant improvement in the system SQ by working on the speaker placement.
Do you have any room for a midbass enclosure in the bottom of the dash assembly, above your feet?
Midbass is not very critical for stage width or height, but is essential for depth. Also, if the pathlength difference is too large, say over 6 inches for the midbass range from 100 to 400 Hz, the imaging will be shifted to the close side.
Mounting them under the dash will help with equalizing path lengths, and leave the kick panel area for the mids, which will have a strong influence on width.
In every case I have worked on in a serious SQ competition car, I have found that placing the primary tweeter as close to the mid as possible yields the best results. With them close together, it is much easier to achieve a coherent wave front from both channels, and minimize comb filtering. The staging and imaging will be much more stable, and easier to tune. If after tweaking the system with this configuration you find that height and width might be improved, you can add a secondary tweeter in the pillar, turned down, and crossed over somewhere above 8,000 Hz, only enough to bring up the corners of the stage, and add some width.
If you have a solid midrange and high frequency stage with good perceived stage height, locating the midbass lower will not affect it.
Comb filtering is a result of the same sound arriving at a point like your ear or a microphone tip, but at two different times. For example, a midrange and tweeter mounted on the dash top pointed at the windshield will result in your ears hearing the direct sound from the speaker, followed a fraction of a milisecond by the reflection of the sound off of the windshield. You get essentially the same sound twice at two different times. The phase differences in the arrival sounds will combine either destructively, or constructively, depending on the frequency and the additional distance/time the reflected sound incurs before arriving at your ear. The result can be some very wildly variating frequency response.
Try sitting in front of a single home seaker with no hard surfaces near you or the speaker. Listen to a song you are familiar with. Then have someone hold a board, large mirror, or some other hard surface so that it creates a reflection that arrives at your ear also. That is comb filtering.