Won’t idle after new muffler

It'll be identified as the "OSAC" port in many carburetor manuals, because starting in '73 many cars had an OSAC valve (Orifice Spark Advance Control) to try to squeak the cars past new-vehicle emissions type-approval tests so they could be legally offered for first sale. In '73 the valve was on the passenger side of the firewall; starting in '74 it was on the side of the air cleaner housing. If your car has one, pretend it doesn't exist—run the hose directly from the carburetor to the distributor. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download; yours is a Holley 1945.