Designation where amount of distribution exceeds rollover contribution
In any case where part or all of the distribution consists of property other than money—
(i)#
the portion of the money or other property which is to be treated as attributable to amounts not included in gross income, and
(ii)#
the portion of the money or other property which is to be treated as included in the rollover contribution,
shall be determined on a ratable basis unless the taxpayer designates otherwise. Any designation under this subparagraph for a taxable year shall be made not later than the time prescribed by law for filing the return for such taxable year (including extensions thereof). Any such designation, once made, shall be irrevocable.