The Secretary of the Interior is authorized to convey to the Utah State Road Commission under such terms and conditions as he may deem necessary such lands or interests in land in lot 3, section 29, township 38 south, range 12 west, Salt Lake meridian, containing approximately four and one-half acres, as are required by the Commission for the realinement and construction of United States Highway 91: Provided, That, in exchange, the State of Utah constructs an interchange of design, type, and location acceptable to the Secretary which will provide vehicular access between the said highway and Zion National Park. Such conveyed lands shall thereafter be considered as excluded from the Zion National Park and the easterly right-of-way line of United States Highway 91, identified as project numbered I-01-1(1), Washington County, Utah, shall become the westerly boundary of the Zion National Park in lot 3, section 29, township 38 south, range 12 west, Salt Lake meridian.