Given that the smell isn't generated by humans (
) Think how much a low pressure area under the car could draw fumes into the cabin. Leaking filler neck would be easily checked, but I would pull the entire tank and check the hoses for the vent and sending unit that exit the tank to the hard lines to the engine compartment. These are almost never replaced when they should be and if you are running the originals with ethanol additive fuels, you have a disaster waiting to happen. Replace these hoses with Gates "Barricade" hose for carbureted vehicles.
Also check for pinhole leaks in the top of the tank where rust gets to rotting the tank from the inside out. Ethanol is your car's greatest enemy , because it creates corrosion when it draws moisture from the air. If your drive you car infrequently, it only gets worse for the fuel system. This is why newer cars use plastic fuel tanks and use polymer plastic fuel hoses with special Viton or fluoroelastome
r rubber products in the system.
Keep looking until you find the leaks, then tune it up.
Pinotosopher, Fuelish Warrior, No dinner without Beano