Pre-PCV Chevrolets have a 'draft tube' or vent tube just behind the intake, it's not really a "provision" for a PCV valve system as PCV valve systems were not in use, and was directed down to the level of the floor pan. Pressure and oil fumes simply vented out of the engine to the air and the passing air would actually assist in scavenging the fumes. I know my 67 327 still has it so I'd assume it was 68 or later when this was discarded in favor of the Positive Crankcase Ventilation system to recycle this blow-by back into the carburetor. If you have a block with this hole, you can cap it with a freeze plug. We used to use the rubber, adjustable type freeze plugs simply because it was easier to size and snug down within the confines of the engine compartment.
Mine has ingeniously been routed via a rubber hose to an adapter on the bottom of the air cleaner (no PCV valve as such) to recycle. Mr. Gasket and others actually made a small 'bung' you could mount to your air cleaner and run a hose from the vent tube to the air cleaner. If you still have the draft tube, it can still be used without a PCV valve system - although it's not very environmentally friendly.
I'm not sure where the oil fill tube comes into play at the front but I'm sure it helps vent pressure as well. Without it (or a PCV system), the rear draft tube may not be enough - can't say for sure. As mentioned, I have the draft tube connected to the air cleaner via a piece of hose plus I have a PCV valve with a newer Edelbrock intake without the oil tube in the front. Sort of a mixture of both worlds.
Dale
