I think the Merc V6 props are a different size to the 75-115 props they sell which means they may not fit your Yam. What we need is the Merc accessories catalogue which gives the sizes of each prop and what it fits. Unfortunately I don't have one anymore.
The boat show in London is next week, might be worth your while going along and asking a few questions. Also try manos, see if he can import one cheaply.
Third option which I always fall back on is Lee Fairweather in Fareham. Fareweather Marine is the business and they have a web site. I wish I knew what he knows about boats. He always has some second hand props too.