The provisions of sections 35 to 45 of this title shall not be construed to modify or amend Title III of the act entitled An Act making appropriations for the Treasury and Post Office Departments for the fiscal year ending June 30, 1934, and for other purposes, approved May 3, 1933 (commonly known as the Buy American Act) [41 U.S.C. 10a et seq.], nor shall the provisions of sections 35 to 45 of this title be construed to modify or amend sections 3141–3144, 3146, and 3147 of title 40, nor the labor provisions of Title II of the National Industrial Recovery Act, approved June 16, 1933, as extended, or of section 7 of the Emergency Relief Appropriation Act, approved April 8, 1935; nor shall the provisions of sections 35 to 45 of this title be construed to modify or amend chapter 307 and section 41621 of title 18.