Couple of thoughts:
both vent lines would have to be blocked to completely block the vent system. Alternatively, the single line that comes out of the vent tank in the fender could be blocked. Check the end of the vent line where it comes out next to the exhaust pipes.
I'd suspect more than that, though, because if you are smelling fuel the tank is not truly sealed. Check inside the tank for rust and crud blocking the fuel pickup screen, or other fuel delivery issues.
George Davis
'69 280 SL Euro manual