Hi and welcome.
Do you need to remove the ABS unit to get the hart in there? If not surely you can add the nut to the thread and then add the plenum? (seems to simple so assume you cannot)
If not I think you need to remove the thread and have it drilled and tapped for a bolt to go in - surely that would give you the room.
OR I wonder if Mark Shead could tell us if it would have a dramatic negative affect to machine a dimple in to the inlet to allow clearance, maybe have a few mill skimmed off the flange face of the inlet where it meets the head as well?