bbd vacuum piston gasket

hey guys i just picked up a 72 dart with a 318 that im trying to get running, for some reason it wont idle at all, i have to keep the revs up so it wont turn off. so i gave it a tune up and im rebuilding the carb right now and its missing the vacuum piston gasket, how does it look like because i bought the rebuild kit and there is two that looks like it could fit in there, one is thicker the other, which one is it?