The Secretary of the Interior is authorized to acquire on behalf of the United States by purchase, at prices deemed by him to be reasonable, the lands needed to complete the Great Smoky Mountains National Park in the State of Tennessee, in accordance with the provisions of sections 403 and 403a to 403c of this title; and the Secretary of the Interior is further authorized, when in his opinion unreasonable prices are asked for any of such lands, to acquire the same by condemnation under the provisions of .section 3113 of title 40