FWIW I believe one of the vent tubes connects to a length of tubing in the tank to the rear of the tank (maybe opposite corner?). I assume so that one of the vent tube ends in the tank would be uncovered most of the time. When I had just a 5" length of hose on my vent tubes and no filter I usually had no problem, but when after filling up the tank sometimes a little gas would slosh out of the vent hoses and drip off the tank. I had a new '70 Nova with an unvented tank and vented cap. The filler neck was low, with cap behind the license plate. I remember getting told all the time that a lot of gas was coming out of my tank upon acceleration. There was a service bulletin in '70 with a replacement cap and I got the cap but still had the problem to a lesser degree. With a low filler neck/cap location to the rear of the tank it's almost unavoidable with a vented cap. The tank mounted vents were the best solution IMO but undoubtedly cost GM more money to produce.