Layout of the gauges, different for street car vs competition car. Not really a big issue, it's nice to have them in the ball park. Look at some pics of the originals or some nicely done replica's to get an idea.
For instance, the Comp cars put the tach where the speedo was on the street cars. So the driver could more easily focus on the rpm's rather than the speedo.
My ERA has a street car dash and gauge placement. However, I swapped the speedo and tach positions, more like what the Comp cars did. I really like it, not how it looks so much as how it's WAY easier to see the tach now. Racing was always about what WORKS well #1 with little or no regard to how it "looks". In this case, it looks pretty cool though.
Dash layout from the FFR web site.
http://www.ffcobra.com/FAQ/dash.html