The older I get the more sensitive I seem to get to fumes of all types ( like raw fuel, exhaust and crankcase).............
I agree the drain tube can allow fumes to enter the cab. Especially if the engine has some blow-by and the tube is inches away from the valve cover slits. I sealed my vent drain tube witha gravity seal that opens when water is present. My engine has a PCV, rebuilt but still sealed all possible areas of entrance of fumes.
Seal all the holes on the fire wall. Hood to cowl seal is direct path to the cowl vent area.
Raw fuel smells are also common and rich mixture makes for strong exhaust fumes compared to modern vehicle. Hot fuel percolation and open float bowl vent makes for raw fuel fumes.
Play with the venting and windows as a experiment to vary the cab pressure. Creating a negative pressure in the cab invites the engine fumes is through the smallest cracks and holes.
Worn heat riser shaft bushings is a common exhaust leak point.
The road draft is totally ineffective at speeds below 25-30mph. Add a PCV is a large improvement on controlling engine crankcase fumes.
