Alumacraft Competitor 165

If it's the standard 90 you’ll probably run a 14 pitch. If it's a command thrust a 15 pitch. 

Mercury Marine has a Propeller selector feature on their home page. Easy to use and helpful. 
​https://www.mercurymarine.com/us/en/propellers/mercury-propeller-selector
As far as mounting holes for the outboard, I'd contact Alumacraft directly or stop in at a dealership to see how a similar boat is set up.

Hooky - Maybe motor height wasn't the porpoising issue then if yours is all the way down, because mine was for sure higher up.  I remember seeing a significant gap between the transom and where the motor would rest if all the way down. 

I was looking to upgrade my gutless 50hp 4 stroke to a 60hp 4 stroke on my tracker boat. Then I read a thread on a hydrofoil. At this point, what do have to lose. I stopped by Cabela's and bought a 300 model which fit my motor. Took it out last weekend and what a game changer. I pulled two kid's on a tube and never knew they were back there. It pop's right out of the water, up on plane and accelerates to 6000 rpm where it should be. That is with the 4 blade prop. Going to try the old 3 blade next. This just saved me several thousand dollars. Totally happy now. The motor had the recommended prop on a sticker on the lower unit. 

I am currently running at 4600 rpms which is 1300 below max. I have a four blade 17 Merc prop and the motor is mounted in the lowest hole (deepest in the water). It also porposies easily when trimmed a little bit. Great hole shot, but would like closer to max rpms which is 5-6000.

Are you changing it due to how it currently rides?  I had the same boat for 3 years and it never was right at full throttle.  I had to have the trim all the way down for it to ride smooth, if I trimmed up even a little it would start to porpoise on me. The dealer acted like they never heard of it before and they sold a lot of those boats, so either I got the special boat or people don't know how boats are supposed to handle.  I was debating having them mount the motor differently but I never got around to it before I sold it.  I did think it was the mounting position though, it looked high to me.