Thanks for your comments.
Rerouting the crank case vent hose to the air filter box was a test, which confirms that oil smoking was indeed caused by a fault in the PCV system. I want to go back to the original set-up, but wanted to confirm that the HC-values in the exhaust gasses were not caused by burning oil.
I did measure crank case vacuum levels @ filler cap, and the pressure was too high (around -3 mbar instead of -15). Don't understand why.
Update: even with the current modifications, HC is still way too high, so probably caused by misfires due to inlet vacuum leaks. Spraying brake cleaner on the inlet gasket area seems to confirm that.
Note that M70 engines have no real positive crankcase ventilation: there is no fresh air input, just blow by gas output. The VCV valve limits vacuum during idling to avoid excessive vacuum and oil suction, like any conventional PCV valve.
Without the stock hose between inlet manifold and crank case, there is overpressure in the crank case, no under pressure. Hence I don't understand the comment about sucking in dirt (there is no more crank case vacuum in my case).