I put the question here simply because I thought it was a fairly simple question, and I had never come across this problem before with my other models.

Making some sort of collar is out the question mostly - finding someone who has a lathe capable of making small bits like that is near to impossible in this country!!
I'm building (assembling) a TF Cessna 310 and looking around here and talking to others, a 12x7 or 13x6 2 blade props are ideal for this engine+model... feel free to correct me, but either of those 2 made sense!

Thanks again..
RN