I have had my 74 Euro Pantera for a good three years and have not filled the tank until today because of concerns about fuel leaks and possible fire hazard. I have kept it at about 1/3. Upon filling it I eventually determined that gas was leaking and I traced it down to a pin hole near the top of the tank. Fuel was spewing straight up like a micro geyser. The tank is not original and was replaced (or at least worked on) in the mid 90's. The hole is very small. Maybe clean it thoroughly and seal it with JB Weld?
The first photo (a tank I am using as an example) shows the approximate location of the leak. The second photo shows the pin hole. The third photo shows the inside of the tank.