Inheritance tax is not deductible on Form 1041. See page 21 of Form 1041 instructions (link below). Toward the bottom of column one.
They are not considered to be administrative expenses. Those amounts are legal fees, filing fees, adminstator fees, etc.
A tax is a tax and cannot be reclassified into another category. On the 706 it is also deductible as inheritance tax not as administrative expenses.