RE: o.s. 120AX "Break in prop"
G'day Mate,
Don't forget you have a very powerful engine, & a 16x8 is not too big, I run a Magnum 120AR with a 16x8, & it run's sweet, & it is not nearly as powerful as the 120AX.
If you are unsure go for a 15x10, but I would use the 16x8 remember don't over rev it, during run in, & it will thank you in the long run.