If you manage to seal the tank well enough to prevent any vapor from escaping, a good fuel pump will collapse the tank, or the engine will stall from fuel starvation. There must be some way for air to enter the tank as the fuel is consumed. I'd suggest fabricating a proper vent system that lets the gas vapors escape to the outside of the cab.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!