Any district which has an existing contract with the Secretary as of October 12, 1982, which does not enter into an amendment of such contract as specified in subsection (a) of this section shall be subject to Federal reclamation law in effect immediately prior to October 12, 1982, as that law is amended or supplemented by sections
230 of this title [43
]. Within a district that does not enter into an amendment of its contract with the Secretary within four and one-half years of October 12, 1982, irrigation water may be delivered to lands leased in excess of a landholding of one hundred and sixty acres only if full cost, as defined in section
of this title, is paid for such water as is assignable to those lands leased in excess of such landholding of one hundred and sixty acres: Provided, That the interest rate used in computing full cost under this subsection shall be the same as provided in section
of this title.