Hi Chook,
The OS 1.60 twin I used on my plane has a totally different method of mounting the prop compared to a Saito 1.80.It uses a threaded sleeve that the prop rides on that screws onto the drive shaft and then a lock nut screws into the center of the sleeve.This lock nut has a threaded hole in the center for the spinner screw.This arrangement probably does not stick out as far as on a normal engine so I can see where you might be running out of room with your set-up.I seem to remember that Tru-Turn may have made some special spinner mounting prop nuts just for this type of problem.It might be worth giving them a call.
Steve