Whenever any State or political subdivision thereof, or any State or Government agency or instrumentality certifies to the Administrator of General Services that any power transmission line determined to be surplus property under the provisions of this Act [sections
1611 to
1646 of this Appendix] is needful for or adaptable to the requirements of any public or cooperative power project, such line and the right-of-way acquired for its construction shall not be sold, leased for more than one year, or otherwise disposed of, except as provided in section
12 [section
1621 of this Appendix] or this section, unless specifically authorized by Act of Congress.