With the recent changes in SVN we now have better distortion compensation, allowing a more precise calibration of your screen setup.
Under the 'Advanced Options' you can enable 'Span Splits' to use all monitors together for a super wide view, then set the distance to you (main) monitor and the 'Arc Ratio' (which defines the radius of the Arc formed by the monitors wrt the driver location. When the game view is zoomed to the correct FovY, the image should appear as if you are looking out through the windscreen/windshield of the car...
The 'seams' in the above image are due to the 'Bezel Compensation', ensuring that when actually displayed the images on each monitor nicely line up with each other.