only such sums as the Congress may specifically authorize by law: Provided, however, That the Congress finds and declares that the national policy set forth in section
1101 of this Appendix requires that there should be authorized and appropriated for fiscal years 1971 through 1980 such sums as may be necessary to construct 300 ships of such sizes, types and designs as the Secretary of Transportation may consider best suited to carry out the purposes and policy of this chapter.