For the purpose of stimulating the acquisition, development, and proper administration and management of State forests and of insuring coordinated effort by Federal and State agencies in carrying out a comprehensive national program of forest-land management, the Secretary of Agriculture is authorized to enter into cooperative agreements with appropriate officials of any State or States for acquiring in the name of the United States, by purchase or otherwise, such forest lands within the cooperating State as in his judgment the State is adequately prepared to administer, develop, and manage as State forests in accordance with the provisions of sections 567a to 567c of this title and with such other terms not inconsistent therewith as he shall prescribe, such acquisition to include the mapping, examination, appraisal, and surveying of such lands and the doing of all things necessary to perfect title thereto in the United States: Provided, That, since it is the declared policy of Congress to maintain and, where it is in the national interest to extend the national-forest system, nothing herein shall be construed to modify, limit, or change in any manner whatsoever the future ownership and administration by the United States of existing national forests and related facilities, or hereafter to restrict or prevent their extension through the acquisition by purchase or otherwise of additional lands for any national-forest purpose: Provided further, That sections 567a to 567c of this title shall not be construed to limit or repeal any legislation authorizing land exchanges by the Federal Government, and private lands acquired by exchange within the limits of any area subject to a cooperative agreement of the character herein authorized shall hereafter be subject to the provisions of sections 567a to 567c of this title.