It can be fixed from underneath, no access from the inside.
It can be leaking at several places. There is a small vent tube that has a rubber hose connection between 2 tubes and the large fill pipe also has a rubber hose connection between the 2 sections. Also, there is an O-ring where the large fill pipe goes into the tank. http://www.trifive.com/garage/57%20Chevy%20Assembly%20Manual/8-3.gif
You may have to take the fill pipe loose from below the fill cap to replace the hoses. http://www.trifive.com/garage/57%20Chevy%20Assembly%20Manual/8-7.gif