Similar story for me... I've recently changed my 72 chall gas tank (having 4 front hoses to a canister, which then has a line running to the engine bay with the charcoal canister). I had a removed the canister many years ago and plugged that line. BUT, I never realized that the only actual venting was from my old factory locking gas cap that would never seal properly at the rubber seal, due to hard rubber... and I'd always put some wound up small rags under the flip cap. BUT further, I didn't realize that those 4 front hoses were actually breaking from hitting the rear of the 8-3/4 differential... and they did leak some fuel only if I had more than a half tank... I run the chall (t/a clone) in autocross and basically always ran with about 1/3 or less fuel, so, the leakage was minimal if ever... and the venting was actually taking place thru the broken hoses and the weak-sealing gas cap/rags. SO... having installed the 70 filler years ago with the top vent tube, I did some research in the OE books... that filler tube runs top vent runs down thru the floor and empties into the frame rail! I've just done this on my newly installed 70 tank; venting is now similar as OE (running a 1/4" rubber hose thru the rear shock opening in the trunk floor, then into the frame rail at the farthest rear, held/shielded under the metal muffler shield (no mufflersb there anyway as they are flowmasters dumping before the diff.) I further just removed the small vent container in front of the tank, AND... I've found a properly sealing gas cap that really seals tight! Now... all this happened because I thought the tank was leaking... in fact, it was only the 4 rubber hoses... and i recently had stalling problems while street driving... gas tank was not venting!!! ANDDD... I'm going to install a check valve at the top of the filler tube vent so fuel will not go out in case of a freak turno ver accident, or else, high g-forces from road course lapping.