Could be either a vent hose leaking or the fuel line leaking
I've had plenty of Pontiacs do this when the tank is totally full. Some only leak a couple drops when you shut them off. Others dribble a puddle as the exhaust heats up the fuel in the tank.
I know that the 1970s have a vent line with a rubber hose in that corner of the tank - not sure if they used the vent on the 1968-69s. But you still have the rubber fuel line and/or return line that will probably be rotten if they're more than a couple years old.
Grab some good, ethanol-resistant rubber fuel line and half a dozen new clamps and jack it up (or get it on a lift, if possible). It should be an easy fix.
Good luck!
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