that was what i was going to suggest.
run the vent line up into the trunk, then into a circular shape like one would do a brake line coming out of the master cylinder, then into the rear frame rail.
or do as NITROUSN suggests in his picture.
the vent must be higher than the tank.
a few years back, my buddy finished up a duster for his wife and had the identical problem. he ran the vent tube up into the trunk as suggested, then back down into the frame rail, and the problem was solved.