After the execution of a contract pursuant to the authority of section 9(d)(1) of the Reclamation Project Act of 1939 [43 U.S.C. 485h (d)(1)] and prior to the commencement of the development period provided thereunder, the Secretary of the Interior is authorized to amend such contract to provide for irrigation blocks, or if such are already provided, to add to or modify such irrigation blocks, as he shall deem desirable to carry out the purposes of that Act.