The larger diameter vacuum line hole in the intake manifold was probably for/from a truck with a hydrovac and a 2-speed-rear selector and wipers, or there might have been a larger vacuum hole in later manifolds?
I have some steel or brass vacuum wiper fittings that fit that larger hole, and some brass fittings for the larger trucks that used hydrovac and 2-speed-rear that fit the larger hole.
A brass reducer adapter should be available at an auto part store or a hardware store.
I'll post photos, if that would help.
