Where a national forest is established under section 471(b) of this title on land previously reserved for the Army or Navy for purposes of national defense the land shall remain subject to the unhampered use of the Department of the Army or Navy Department for said purposes and nothing in this section or section 471(b) of this title shall be construed to relinquish the authority over such lands for purposes of national defense now vested in the department for which the lands were formerly reserved. Any moneys available for the maintenance, improvement, protection, construction of highways and general administration of the national forests shall be available for expenditure on national forests created under this section.