The Secretary or Administrator shall not guarantee the principal of obligations in an amount in excess of 75 per centum, or 871/2 per centum, whichever is applicable under section
1274 of this Appendix, of the amount, as determined by the Secretary or Administrator which determination shall be conclusive, paid by or for the account of the obligor for the construction, reconstruction, or reconditioning of a vessel or vessels with respect to which a security interest has been conveyed to the Secretary or Administrator, unless the obligor creates an escrow fund as authorized by section
1279a of this Appendix, in which case the Secretary or Administrator may guarantee 75 per centum or 871/2 per centum, whichever is applicable under section
1274 of this Appendix, of the actual cost of such vessel or vessels.