Any proposal with respect to the construction of a project which has theretofore been authorized for construction under the Federal reclamation laws shall be made in like manner as a proposal under section 422d of this title, but the Secretary may waive such requirements of subsections (a) and (b) of section 422d of this title as he finds to be duplicative of, or rendered unnecessary or impossible by, action already taken by the United States. Upon approval of any such proposal by the Secretary he may negotiate and execute a contract which conforms, as nearly as may be, to the provisions of section 422e of this title.