On my ZDZ 80RV (silver head) I only have the 6 smaller bolts (with flat washers and split locking washers).  There is a center shaft, but no nut or even threads that stick out enough to bolt on a prop.  I tighten the 6 small bolts down using a criss cross pattern to 20 inch lbs, then retighten again to the 20 inch lbs.  I then tighten them up to 30 inch lbs, again using the criss cross pattern and then again to the 30 inch lbs.  I put a drop of machine oil and the threads.  I have never had a problem with them coming off or breaking, and any tighter seems to really start to squish the prop hub.  Tightening to the 30 inch lbs puts a little bit of squish on the wooden prop hubs, but not very much.  I do check it after a weekend of flying and they are still good.  When every I replace a prop or take the bolts off, I try to replace the split locking washers (but this does not happen every time but I try too).
If I remember correctly from a previous thread, someone mentioned that you either use the large single nut or the 6 smaller bolts not both.  A copy of the manual that I have does not mention the large single nut at all.