Determinations and decisions provided in this Act to be made by the Administrator or other agency head shall be final. Such determinations or decisions may be made with respect to individual purchases or contracts or, except for determinations or decisions under sections
253,
253a, and
253b of this title, with respect to classes of purchases or contracts. Except as provided in section
253
(d)(2) of this title, and except as provided in section
121
(d)(1) and (2) of title
40 with respect to the Administrator, the agency head is authorized to delegate his powers provided by this Act, including the making of such determinations and decisions, in his discretion and subject to his direction, to any other officer or officers or officials of the agency.