Gas Cap question

My 70 Duster as originally equipped with the Californa Emmisions (N95). At some point before I bought it the gas tank was replaced with a different type. One that does not have the 4 vent tubes connected to it like the original did. The lines were just cut. The breather in the passenger side valve cover is still original but the line that goes to it is just there for show. Since the system is not as intended with missing vent tubes and all should I be using a different type of gas cap? I get some serious vacuum sound when I remove the cap on a warm day especially.