PCV question for 66 c-10 230 inline six

[Heater63]

On that engine you would typically have a breather cap and the PCV valve in the valve cover. You need a grommet for a tight seal around the PCV valve.
If you run a plain oil filler cap, your PCV system will not work. You must have a source for air to enter the crankcase, that is what the breather cap is for. Clean air goes IN through the breather cap (or pulled from the air cleaner housing), crankcase air/fumes go OUT through the PCV valve/hose to the pcv port on the carb.
On a V8, the air inlet is on one valve cover, the pcv on the opposite side. On the I6, the breather cap is on the front, the PCV on the rear.