Did you replace the sending unit gasket? The new style rubber (PVC, maybe) ones have under-sized screw holes for a 100% seal as compared to a dried out original cork seal.
Old sending unit gaskets are notorious for leaking fumes which will rise up, flow out, and collect on top of the tank.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission