Garret,

I'm sure the cap is vented. It wouldn't be obvious unless you turned it over and examined it. All carburetors have some type of a opening between the fuel bowl and atmosphere. After running the truck and shutting it off, latent heat can cause the fuel in the bowl to expand and dribble into the intake manifold. This can cause vapors to rise and exit the carburetor giving you the gas smell.

Does the truck have a air cleaner on it. I wonder if it has the phenolic heat spacer between the manifold and carburetor? Unless it drips and makes a mess, sometimes it's part of what comes with owning one of these old girls.

Stuart