Performance Gauge

if its the same as used on the 2nd generation cars, the vacuum hose is pretty small.
Redfish and I were posting about this in 2020. I think the hose fitting is 1/8".
If you can't find a NPT fitting with that size hose nipple, there are in-line hose to hose fittings that will let reduce the size.

The baseplate of the Holley carb ought to have a 3/16 manifold vacuum fitting on it. So that's an alternative but will need the reducer from the vauum hose that fits on it to the hose connecting to the vacuum gage.