Have you compared the 'Pixel Pitch' yet? If they have different pitches (the spacing of the pixels) then the monitors won't be the same height. Think of it this way, 19" + 17" LCD (non-widescreen) both have 1280x1024 resolution. If you tried using a 19" as a center screen with a 17" on either side of it, it would look horrible because the outer screens would have a shorter height.
Personally, I replaced my 1600x1200 20" LCD with a 1920x1200 24" LCD. I was hoping that perhaps I could add another 1600x1200 20" screen later to do the same thing you are talking about. I never bothered looking into it because of how different the pixel pitches were.
Sorry to say that I don't know if it is possible. But, the FIRST thing I would check is the pitch, because if they are different, you won't want to go any further.