The vent needs to be there to let air escape from the tank as you fill it. Otherwise the gas will back up into the fill tube and cause the pump nozzle to trip itself off. Use the nipple on the opposite side of the tank from the filler and attach a piece of heater hose, etc. to it and route it as high above the tank as you can. Stick a piece of steel wool or something similar into the end of the tube to prevent bugs or other debris from getting into the tank. If you've got stake pockets in the bed, that's a good place to put the vent tube. You can get it a foot or more above the top of the tank that way.
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!