aussie, the width between the two clutch plates and the nuts that locate them, is set by a series of parts across the center, including some shims. I suggest you take another look at this thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=21336&page=1
In that thread aldot and Murray both talk about adjusting the clutch plate clearance by using the nuts at the sides of the clutch pack. aldot says the washers in that area are crushable, so you need to tighten the nuts just the right amount to get the proper clutch plate clearance. Murray seems to be saying that his washers are crushed further than he wants them to be, so he is now using loctite on the thread of the adjusting nut to hold it in the position he requires.
I haven't seen one of these clutch packs, and the Greenfield parts list doesn't show how it works. I think we need aldot, Murray or Deadly Dave to clarify the position for us both. If my reading of that other thread is correct, you just need to apply more torque to that nut to crush your washer stack and get the correct clutch adjustment. You would need to end up only moving the nut on the forward clutch side, not the other (reverse clutch) nut. Let's see if one of the experienced guys can clear this up for us.