Leaking gas tank filler tubes ?

I believe the issue is with the filler tube. The early cars did not use a vented cap. There is a vent tube that connects to the top of the filler tube, and hangs down beside the fuel tank. When fuel sloshes around, some will spill out the vent, usually only on a full tank. I noticed that if I tried to squeeze a little more into my tank at fill up, I would end up with a little puddle under the car.
The easy fix for this would be to block off the vent tube, and use a vented cap.