Installing Struts

Is it because the shaft is turning too when you try to tighten it? If it is (I had the same problem), you can take a large flathead screwdriver and stick in the slot on top of the shaft to hold it still and tighten the nut with a wrench. I had to do that on both my 90 and my 04.
 
Shocks have flats and the struts have slots. I use a leverage-bar to hold the strut via the slot.

Good luck.