the mass of mcgowan is pretty N/A for 39s imo. You could use it - I just don't really see any point when mallet of ZF is better in just about all scenarios. There are some situational circumstances where the BOA mace might be better for a few seconds (WF procs, whirlwind) but imo the fast speed on ZF for very similar damage mean that on the whole ZF mace will be better more often ("more" wf procs, more attacks to proc flurry, etc.).
as for offhand - vanquisher's sword is really really good. I guess if you were a combat rogue with +100% offhand damage talent then the DPS from mass of mcgowan could be best in slot.