I just saw your post.
I have an Apache that I converted to brushless.
Here is my setup:

Feigao 540 10L 2458kv
3S 11.1v 500mah lipo
Hacker 70-3p 70amp ESC
converted to .130 flex cable
separate strut and offset rudder
Y534 plastic prop.

This conservative setup looks like it is pushing 30mph maybe more and nothing gets over 110 degrees. An L sized 540 is a little overkill for a boat this light, but it will last forever.

One thing you will notice once you start getting some speed out of this boat is that the steerable outdrive will cause the boat to flip if you try anything but huge sweeping turns. That is why I switched to a separate prop strut and rudder. It is much more controllable.
Your turn fins will help to a point, but they are more of a bandaid than a fix. It all depends what you want out of it. If you want to go fast in a straight line and take it easy in the turns it will work, but with fast turns, you better have a rescue boat at the ready.

The plastic U joint will not hold up too long under brushless power either, that is why I converted it to a .130 flex cable as well.

Also, if you haven't done it yet get some pool noodles to put in the bow, because when it flips you want it to float, for obvious reasons.
Make sure also to tape the hatch to make it as waterproof as possible.

It is a fun boat to modify and it is a good learning platform.
Let me know if you have any further questions.
Brad