There are hereby authorized to be appropriated such sums as may be necessary to carry out the provisions of this subchapter, but not more than $61,610,000 plus $15,500,000 shall be appropriated for the acquisition of lands and interests in lands. There are authorized to be appropriated not more than $58,000,000 (May 1971 prices) for the development of the recreation area, plus or minus such amounts, if any, as may be justified by reason of ordinary fluctuations in construction costs as indicated by engineering cost indices applicable to the type of construction involved herein said total development ceiling to be reduced by $10,000,000.