Bob in Ct.
If you have a race engine in a race car, you don't need a PCV. If you have a street engine in a street car, you need a PCV.
PCV (positive crankcase ventilation) valves have been installed on virtually all engines since about 1963. It puts the inside of your engine under a slight vacuum, removes the
oil vapors generated in the crankcase from your engine, injects this under the carb base into the intake where the vapors are burned. And it only does this when the engine is under high vacuum or low load so it doesn't mess up your power enrichment.
There should be an air inlet breather on the drivers side valve cover. With the engine running, if you put your thumb over this hole with the breather out, you can feel the vacuum. The air then circulates through the engine to the passenger side valve cover where the PCV pulls the air and
oil vapors out through a hose to the intake.
Go to the Fram or Purolator book alongside the rack of PCV valves, find the part number for a early - mid-70's PCV for a Ford 351W. If there are a couple to choose from, the one you want is the one with the little plastic 90 deg elbow on top with a 3/8" hose fitting.
Remember to jet your carb primaries about 2 steps richer to make up for the extra air the PCV lets in. Secondaries shouldn't be affected.
Can you get by without one? Sure. But your engine, and engine compartment will get dirtier (oilier) faster.