The PCV valve needs a vacuum source.
The original setup had a large tube connect from the air cleaner to the hole in the front side of the valve cover. The valve cover hole at the rear held the PCV. A hose from there went to a vacuum source at the manifold or carb base, depending on the year and carb used.
The idea is to have fresh air that is filtred by the carb air filter, enter the front of the valve cover. That fresh air enters the engine as the the crankcase fumes are drawn through the PCV valve to the intake manifold and burned with the air/fuel.
I modified the air cleaner to fit a 4bbl, but the hose to the front of the valve cover, is the original. This is a '67 with oil bath air cleaner. Yours probably had a dry filter directly above the carb.
The down slopping port you have the black plug/cap on, is for a vacuum cannister. That replaced an idle vent used on older carbs. I don't think it should be plugged completely. I would put something inside the end that would allow it to breath, but can be pulled out for cleaning.
PVC is Poly Vinyl Chloride for lawn sprinklers.
PCV is Positive Crankcase Ventilation
