In furtherance of the purposes of the Act of March 4, 1931 (46 Stat. 1522), the Secretary of the Interior is authorized to acquire by purchase, exchange, donation, with donated funds or otherwise by such means as he may deem to be in the public interest, lands and interests in lands adjoining or near the Fort Necessity National Battlefield site which in his discretion are necessary to preserve the historic battleground, together with not to exceed 25 acres at the detached Braddock Monument: Provided, That the total area acquired pursuant to sections 430pp to 430tt of this title shall not exceed 500 acres, except that in order to avoid the undesirable severance of parcels in private ownership such parcels may be purchased in the entirety.