By the mid '60's, Chevy had introduced crankcase ventillation on its SB engines. The earliest systems were "open" systems that drew air in from the breather in the oil filler cap at the front of the engine, through the engine, and then from the engine through the PCV valve to the base of the carb. 283 and 327 engines in the mid '60's had a fitting at the back of the block, near the distributor. A hose was connected to this fitting at the back of the engine. The hose also had an in line PCV valve with the other end of the hose connected to the base of the carb. Scavenging air flow was through the breather in the oil filler cap at the front of the engine, through the engine, out the engine at the back, through the PCV valve, and into the base of the carb. This system is an open system in that funes can vent out the oil filler breather. You can readily see this, especially just after the engine is turned off. My '66 Chevelle (283 engine) has this arrangement.
When the transition was changed to a "closed" crankcase ventillation systems, the oil filler cap no longer had a breather and was changed to a sealed cap. The connection at the back of the engine was eliminated with the PCV valve being moved to LH valve cover. Scavenging air flow was through the seperate filter in the air cleaner, into the engine, out through the PCV valve, and into the base of the carb. In 1969, the oil fill tube was eliminated at the front of the engine with the oil fill sealed cap moved to the LH valve cover. My '69 RS (327 engine) has this arrangement.
As previously stated, you have to be able to draw filtered scavenging air into the engine in metered quantities, which varies with engine vacuum, and exhaust it into the air fuel mixture at the carb. If your engine is supposed to have an operative PCV system and you "plug" it, all kinds of mischief can occur including corrosion, dillution of the oil, and sludge.