For the purpose of securing any rights of flowage, or obtaining title to or possession of any property, real or personal, that may be necessary or may become necessary, in the carrying out of any of the provisions of this chapter, the President of the United States for a period of three years from May 18, 1933, is authorized to acquire title in the name of the United States to such rights or such property, and to provide for the payment for same by directing the Board to contract to deliver power generated at any of the plants now owned or hereafter owned or constructed by the Government or by said Corporation, such future delivery of power to continue for a period not exceeding thirty years. Likewise, for one year after