Any contract made for financial assistance under the United States Housing Act of 1937, as amended [42 U.S.C. 1437 et seq.], may be revised so as to provide that the project involved will be assisted for any of the purposes of this subchapter. The Department of the Navy or Army or the Air Force or the Secretary of Housing and Urban Development, in the administration of any project developed for the purposes of this subchapter, shall fix rentals for persons engaged in national defense activities and their families which will be within their financial reach, and the Secretary of Housing and Urban Development, in any contract for financial assistance or any lease of such a project, shall require the fixing of such rentals. Proje