Unless the decedent directs otherwise in his will, if any part of the gross estate on which tax has been paid consists of proceeds of policies of insurance on the life of the decedent receivable by a beneficiary other than the executor, the executor shall be entitled to recover from such beneficiary such portion of the total tax paid as the proceeds of such policies bear to the taxable estate. If there is more than one such beneficiary, the executor shall be entitled to recover from such beneficiaries in the same ratio. In the case of such proceeds receivable by the surviving spouse of the decedent for which a deduction is allowed under section 2056 (relating to marital deduction), this section shall not apply to such proceeds except as to the amount thereof in excess of the aggregate amount of the marital deductions allowed under such section.