When filling, the over fill tube will drip behind the quarter panel when you fill up to the point the fuel rises up in the filler tube. Why you have fuel in the well ( it should have a drain also ) is a mystery to me. All the old caps were vented to prevent suction problems before the EPA got involved and fed those fumes to a canister that then went to the
carb or
intake manifold to be burned.
Try leaving the cap at it's first notch so it's on but not loose and see what happens. It could be as simple as the wrong cap.
My '57 likes to burp back and splash out the filler. I've learned to stand back and fill as slowly as possible. I guess back then the pumps didn't put out that much volume.