For the purpose of including within Whitman Mission National Historic Site, Washington, certain properties that are of historic significance in connection with the site area and which are needed to provide suitable monument facilities, the Secretary of the Interior is authorized to procure not to exceed fifty acres of land adjacent to the existing site and a right-of-way thereto from United States Highway 410, using therefor any land acquisition funds available for the purposes of the national park system, such property to be acquired in such manner as the Secretary shall consider to be in the public interest. Following the acquisition by the United States of land for addition to the site pursuant to this section, such addition shall be effective in each instance upon the publication of notice thereof in the Federal Register.