The oil tank vent should connect to the engine rear fitting.
Consider what happens in the oil tank with heat/cold cycles. The air in the tank will expand and contract. Upon contraction, it draws in humidity from the atmosphere and the water then condenses on the tank walls and waters the oil. Only prolonged heat will evaporate the water and send it out the vent tube, or into the engine oiling system.
The same thing happens inside the engine crankcase. But it takes far longer to fully heat the oil tank and disperse the water collected on the tank walls. And if the oil tank is routed through the engine crankcase, a far longer, delayed path for humidity to reach the oil tank.
Ideally, some less than atmospheric pressure is applied to the crankcase to help evacuate the water vapors (from both oil tank and crankcase).
The type of open crankcase ventilation system the early SOHC4 had was abandoned by most vehicle manufacturers with the advent of the PCV (Positive crankcase Ventilation valve (invented in WWII and first installed on production cars in 1961, according to WIKI). This closed the venting system so expelled vapors went through combustion before being released to atmosphere. The pathway chosen was on the manifold side of the carbs, as most engines had a common plenum at that point of the engine system's single carburetor.
In 74, Honda began closing the engine ventilation system by routing the vapors back into the induction system before entering the carbs. The only common plenum available was the air filter box. Though not as deep a vacuum as what most PCV systems utilize, the air filter box does have less than atmospheric pressure in it while the engine is running, and this helps evacuate engine water vapors (as well as other carcinogenic vapors) through the combustion cycle for elimination.
For those wishing to "modernize" their vintage bikes, note that nearly ALL modern engines incorporate a PCV system.
As a side note, there have been successful adaptations of adding an evacuation pump to the crankcase of racing engines. Running negative pressure in the crankcase helps seal the ring fit between piston and cylinder, reducing blow by and actually increasing compression in the combustion chamber for a more power output result. Honda's SOHC4 recirculator won't approach that, of course. But, it should help keep water out of the crankcase to help engine longevity.
Cheers,
