/6 with headers???

hello everyone, I have a question. Right now my father and I already set up the headers and attached the manifold. on the manifold on the bottom there is an square opening and I was wonder if you had to cover the hole with something like a metal plate? I have a 1973 Plymouth scamp Slant 6 in near end project stage (almost finished in engine). The manifold that holds the carburator on the bottom side. just curious, I don't want the fumes or anything else coming out of the bottom, thanks you Guys / Gals .