The workmen’s compensation act shall not be construed to apply to business or employment which, according to law, is so engaged in interstate commerce as to be not subject to the legislative power of the state, nor to persons injured while they are so engaged: Provided, That the workmen’s compensation act shall apply also to injuries sustained outside the state where: (1) The principal place of employment is within the state; or (2) the contract of employment was made within the state, unless such contract otherwise specifically provides: Provided, however, That the workmen’s compensation act shall apply to all lands and premises owned or held by the United States of America by deed or act of cession, by purchase or otherwise, which is within the exterior boundaries of the state of Kansas and to all projects, buildings, constructions, improvements and property belonging to the United States of America within said exterior boundaries as authorized by 40 U.S.C. § 290, enacted June 25, 1936.