You don't allocate the step-up to select holdings, you apply it to all holdings. The formula is (Date-of-death fair market value + Old basis) / 2 = New Basis) For instance, if you owned stock XYZ worth $5,000 at the time of your wife's death, and your basis was $1,000, you new basis is $6,000 / 2 = $3,000.
