Hi again,
I don't know where the RPM sweet-spot for an FZ70 is, but it cannot be more than 10,000 or so.
What that means is that you can run up to a 13" diameter prop without problem.
My guess (and this is only a guess) is a 13-7 prop. Try it for a start.
Hello those of you with experience with the FZ70 ... what size prop are you running and what is your ground RPM?
If you could tell us whether you think you are loading the engine heavily with your setup, or not, please add that information as well.
I did see my buddy Matt run a YS 63 on a 13-7 and it was just a teeny bit lugged down, but handled it well. No, I don't remember the RMP.

Regards,
Dean