Either plug the vent and use a vented cap or use a closed cap and use the vent. Your choice. The guys at the drag strip use rollover valves and run a piece of hose to the outside of the car if the cell is enclosed in the car. If you don't want to run a check/rollover valve, just run some hose to the underneath of the truck and attach a small air filter element to it to prevent dirt from entering the cell, but still allowing the cell to breathe.
http://www.atlltd.com/technical.htm
Click vent valves.