RE: sport 40 prop
The X-646/3 is a great place to start but the engine might take more, it will let you know.
I haven't seen the insides of the Aeromarine Sport 40 but you want as much weight forward as you can. mount the fuel tank on the right side if possible and even better if you can get it down into the right sponson.
If the radio box is movable, try to get it as close to the engine as you can but not so close that you can't get in to wrench on the flex cable coupler.
Got any pictures that you can post? I'd like of see the inside of it.
Seeing how you're in IMPBA territory, only the drive dog on back can be exposed when looking down from the top. Some people mount the strut on the floor of the boat. On the one that I built, I mounted the bracket on the inside of the transom. I just didn't like the idea of having the bolt heads hanging down out of the bottom. Button-head allen drive bolts would be cleaner underneath though if you go that route. They are harder to find in Stainless steel though.