Some later experiences with Aurelias confirm the exhaust issue - in s. 4 B20s, it comes in from the rear boot lid. There is a negative pressure area there. I don't think this is uncommon in Italian cars, as there are several aftermarket exhaust turn-downs, or diverters - and in the 1980s, the Alfa spider came with such from the factory. So too, the correct fitting of the seal around the boot lid is key - I think its mounting is a bit tricky, and many times its done incorrectly. If memory serves correctly, it mounted on the body, not the lid.
Also, later B20s have their fresh air intakes (for the cabin) in the footwells mounted in the firewall - and can bring in fumes from the engine bay. Earlier B20s (like our s.2) have the fresh air intakes in front of the radiator and then ducted back to the cabin, ensuring cleaner air (which is successful). And the little fins on the back of the body somehow affect the air flow, so there is less backdraft of exhaust fumes. Overall, the interior of the car has less fumes, but, to be honest, is not to the same level as modern cars. We tend to forget that back in those days, a ride in the car meant experience with some amount of smells. Its just good to have less...