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Attach small hose to vent tube and run to a point higher than any full fuel level point so that fuel will not reach that point and dribble out.
Another choice would be to plug that vent and use a vented gas cap.



The vented cap on a 68-70 B body is a bad idea because the filler tube is behind the license plate in the bumper , a fast take off has the gas spilling on the ground behind you.

There are 2 vents on the tank , do you have the short line connected to the right one and is it mounted up high in the shock crossmember ? Is the other vent line open and not clogged ?