When title to the site or portion thereof at Fort Stanwix, in the State of New York, together with such buildings and other property located thereon as may be designated by the Secretary of the Interior as necessary or desirable for national monument purposes, shall have been vested in the United States, said area and improvements, if any, shall be designated and set apart by proclamation of the President for preservation as a national monument for the benefit and inspiration of the people and shall be called the "Fort Stanwix National Monument": Provided, That such area shall include at least that part of Fort Stanwix now belonging to the State of New York.