I am going to leave the road draft tube open as hotrod suggests so that the engine can breath.
Most road draft tubes have a rear-facing tapered cut at the end of the pipe that is supposed to create a low pressure area from the air passing under the vehicle at speeds above 30 MPH. That's why the tube is long enough to hang down into the airstream. That's how crankcase ventilation was accomplished in the days before PCV systems were developed. The slots in the valve cover on the 216 or the ventilated oil fill cap with the horsehair packing in it gave air an entry point into the top side.
I'd suggest plugging the end of the road draft tube to prevent having two opposing vacuum sources, and drilling about a 1/2" hole in the tube about 6" up from the bottom, out of the airstream. That will allow air to enter the crankcase without creating a resistance to the airflow caused by the low pressure area at the bolonga-sliced end of the road draft. Sneaky guys, those oldtime engine designers!
Jerry
