I know it's a pain but my first step would be to double check/replace all the rubber fuel hoses under the bonnet.
The metal pipe that brings fuel to the engine bay is usually OK but worth checking the connections - I'm sure you have, but worth looking.
If we assume that the carb isn't doing anything funny and that all under the bonnet is fine then I'd double check the condition of the pipework at the lift pumps at the fuel tank.
I had no end of issues with that (to begin with) on my old HPE VX then again with my coupe VX but to be fair it was rare for the fumes to make it into the cabin.
The final possibility that springs to my mind at the moment (and it's been a few years since I did anything Lancia so bear with me) is an inline fuel filter - if you've had one fitted?
If you have, worth checking it over carefully. I replaced the plastic one orginally fitted to my VX coupe with a generic metal filter as the plastic ones seemed to basically come apart under a VX bonnet. They shouldn't, but did.