Upon recommendation of the Secretary of Agriculture, the Secretary of the Interior may patent to the State of South Dakota not to exceed one thousand six hundred acres of nonmineral national forest lands not otherwise appropriated or withdrawn within the areas of Norbeck Wildlife Preserve: Provided, That the State of South Dakota conveys to the Government good and sufficient title to other lands of equal value owned by the State and lying within the exterior boundaries of a national forest in the State of South Dakota and approved by the Secretary of Agriculture as equally desirable for national forest purposes, the lands thus conveyed to the Government to become a part of the national forest. This shall not operate to restrict any selection rights which the State may have or may be hereafter granted, excepting as to the specific lands conveyed to the Government under authority of this section.