The TR3 ventilation is made so air from the radiator is forced into the valve cover vent under slight pressure. The air circulates through and around the internal engine, and then is pulled out the lower road vent tube by a slight vacuum.
The later Positive Crankcase Ventilation (PCV) system used by all modern cars pulls air from inside the intake filter, circulates through the engine, and is pulled into the intake manifold, so that all oil and contaminants from the engine are burned in cumbustion.
In answer to your question, you need a circulation of air in the engine, to remove blow-by. That requires a way for air to enter and a way for it to exit. One vent is probably not a good idea...but it will work. You will just force blowby containing oil all over the top of your engine.