I am not certain of this but.. I think that is for a return line. The suction should come out of the top of the tank.

From back in the 60's sometime. All tanks had to feed from the top for "safety" considerations. Even older truck fuel tanks had to be converted from bottom supply to top, local DOT would impound your truck if caught on the highway with "old style" tanks.

Check around the sending unit and see if there is another opening about the same size...