The road draft tube was used on pre-PCV engines. It's a large diameter tube coming out of the crankcase near the distributor, and turning down into the slipstream below the engine. Above 30 MPH, air passing past the end of the tube creates a vacuum, and draws air out of the crankcase. Slots in the valve cover, or an oil fill cap with a mesh filter allows air to be sucked into the valve cover. The PCV system was used from the late 60's on up, and resulted in a cleaner-running engine because crankcase vapors were drawn into the intake and burned in the cylinders. Engine wear decreases dramatically, since dirt isn't drawn into the engine if the PCV system uses filtered air for ventilation. Your engine can be adapted for positive ventilation pretty easily, but it's frowned upon by the restoration purists.
Jerry
