slant 6 holley carb?

A photo would help.....Is it a Holley 1920? 1945? If the fitting in question is the bowl vent for the charcoal canister and your car doesn't have a charcoal canister, you should be able to plug it. If it's some oddball model, it might need the vent open so it doesn't develope a partial vacuum in the float bowl. Sounds like it has a rubber plug in it now and has been running ok for many years now? If so, keep it plugged like it is.