The 6.7 damper is 1/4" thicker than the OEM 5.9 (not including integral reluctor wheel), but it's 3/4" larger O.D. - even with the tapered shoulder - won't clear the 5.9 fan pulley w/o removing ~.080" of the shoulder's peak... you can also move the idler/fan pulley bracket.
All parts interchange by themselves, but as you've found they don't always mix & match together.