It could be a slight burr on one of the threads. I would apply a light coating of grease on all the hub threads then gently thread the Dura Ace freewheel on by hand using the correct splined removal tool by hand to work it back and forth.
If the 7400 freewheel body has been exposed to any corrosion (which is entirely possible), I would take a clean shop cloth with some WD-40 on it and just work it back & forth on the freewheel body threads in order to remove some light corrosion or thickened old grease. Then re-try.