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I think SIL's motor is a 1999 , but not sure. It is 1.5 hours from me and 2 hours from him so that won't help. Look at my recent post about mounting height. Joe has posted the link to mounting height terminology.

I think you should mount 3 holes up according to that link. That may not be true if using an aluminum prop though.
I would seriously consider stainless. I used alum for years, just because that is what the first boat came with. After I bought a new Yamaha in 1985 [ in those days they came with SS props] I have never had anything else and I have never even had to get a prop touched up.

This is despite running aground in the one boat almost every use to get in and out of the creek. I am not hitting rocks of course, but the old issue of ruining gears is apparently solved by the hubs employed nowdays.

Others on here will hopefully chime in about that.

On another note, I just paid $75 for a 17K Yamaha SS last weekend on Craigslist. I am considering a new project boat and motor and figured the price of a prop that I might need was too good to pass up.

Maybe you can find something used for your first try and then buy new and keep it as a spare if correct.