What's the deal with...

early A-bodies leaking fuel down the driver side quarter panel? I see a lot of photos of A-bodies with tell tale stains and paint wear from this problem. Does the seal for the fuel cap dry out or something? The tank sits quite a bit lower than the filler hole. Do people just overfill the tank to the point where the fuel level is all the way up the tube and it sloshes out? Any ideas on how to prevent this? Someday when my car is ready for paint, I don't want gasoline spillage all down my new clear coat. Thanks for any helpful tips on this subject.