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

By your measurements it would look like you need 7/16"
I would try an Ace typ hwd store as they typically have more add ball parts.

Also try ebay. If it exists in the world it will be there.

Also check in the plumbing area where they have misc fittings in home Depot usually by the sprinkler area