The Pontiac vacuum gauge and tach share the same housing (aka: cup).
On one of my 1965 GTO's I took a tach out of a 2+2, took the tach out of the housing, put some tape on the tach body near the bezel, removed the blank in my dash bezel where the clock would go and inserted the tach. It was a perfect fit with that tape around it. Today, you would probably use silicone. The 1965 and 1966 openings are the same and I think a vacuum gauge in that location would look kinda cool.
You can take the vacuum gauge housing and either sell it, as there is a market for those, or stick it in a junk drawer for some day.