2536771 CAP Intake Manifold Port Plug (FOUND)

Does anyone know what size plug I would use in this CAP 4bbl intake manifold 2536771 the large port hole that I am not going to use? It's right next to the vacuum performance gauge port that I am going to use. Every plumbing shop or hardware shop in the area has either 1/2" which is too big or 3/8" which is too small.
Thanks
Bob