Francis,
I guess the first question is what type of roadster. We run a Gas/Fuel roadster on a tube frame. The step pan can be mounted inside the frame or to the bottom of the frame no lower than the bottom of the frame + material thickness. The step pan cannot cover the rear end, The rule of thumb most roadster guys go by is 10" before the rear end the step pan starts going up. Past the rear end you can continue with the step pan as long as it stays within or mounted to the bottom of the frame rails. These are the rules for gas/fuel roadster as I interpret them. If your running modified or street the rules change. There is a roadster commitee that you can ask on the salt. Just remember that nobody is going to tell you your car is illegal unless you ask, and nobody will protest until you go fast. Hope this helps and I remeber this is just how I interpret the rules. Have fun, go fast
Dallas