well when I put a vented cap on my 61s with a vented tank fuel pours out in corners.
correct on the line. it comes out of filler neck, crosses back over the top of the tank, gores up into the passenger side pillar to the roof, turns back down and exits through the floor.
the reason it will leak on a vented cap is because the vented cap will leak under enough pressure. having the vent tube ads to that pressure. put your finger over a straw and take it out of your drink and it will be full of liquid. let your finger off and it drains. the vent tube allows that extra pressure on the cap so it must be non vented.