Upon a written finding by the head of the contracting agency or department that the inclusion in the proposal or contract of the representations or stipulations set forth in section 35 of this title will seriously impair the conduct of Government business, the Secretary of Labor shall make exceptions in specific cases or otherwise when justice or public interest will be served thereby. Upon the joint recommendation of the contracting agency and the contractor, the Secretary of Labor may modify the terms of an existing contract respecting minimum rates of pay and maximum hours of labor as he may find necessary and proper in the public interest or to prevent injustice and undue hardship. The Secretary of Labor may provide reasonable limitations and may make rules and regulations allowing reasonable variations, tolerances, and exemptions to and from any or all provisions of sections 35 to 45 of this title respecting minimum rates of pay and maximum hours of labor or the extent of the application of said sections to contractors, as hereinbefore described. Whenever the Secretary of Labor shall permit an increase in the maximum hours of labor stipulated in the contract, he shall set a rate of pay for any overtime, which rate shall be not less than one and one-half times the basic hourly rate received by any employee affected: Provided, That whenever in his judgment such course is in the public interest, the President is authorized to suspend any or all of the representations and stipulations contained in section 35 of this title