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
What do you think of the mid-70s evap systems that vent the tank into a charcoal canister with a hookup to the carb to draw the vapor back off and burn it?
