Getting fumes in the cab real bad

Buddy drove a 396 C10 pickup from ND to Cali with a bad fuel cap gasket. I guess the fuel tank is behind the front seat or something and he had a headache for a week after that stinky road trip. Didn't ask what the fuel bill was. Gas fumes sounds like a fuel leak. Exhaust is different...quite a few gaskets ie. Steering column. Firewall grommets, exhaust leaks.......