be sure to leave some type of ventilation in the cabin for any incoming air to escape. if the cab gets pressurized it can slow your et, even with everything blocked off. make a pass with all windows up and everything closed, then make one with the back windows cracked for air to escape (not too far or the starter may back you up!) and compare the times. i know guys that go as far as to install a belly pan under the engine bay to cut drag.