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I guess I am not understanding this totally...

A Standard transom is 17 inches... on a 13' model ?
If you add 5 inches to it, that makes 22 inches...

Not all engines are exactly the same length for long shaft motors.
They all can vary slightly.

You really need to run the boat to see if the engine is too high or too low.
In this case, I would start off by having the engine all the way down.
If you get too much spray from the cavitation plate, then the engine is too low and need to be raised up a hole or maybe two.
Major concern is not to have the engine too high where it can't pump water to cool the engine down.

After the height is adjusted properly, then you should see if the prop is the correct choice.
You can view this thread about prop pitch and RPM's on the tachometer.
http://www.whalercentral.com/forum/vi...;pid=67402
and this one:
http://www.whalercentral.com/forum/vi...post_67250

Basically the cavitation plate should be about 3/4 of an inch to 1 1/2 inches above the bottom of the keel.
Again, not too high or the engine water pump won't pick up the water to cool the engine and could cause major damage.
Too low, and your performance suffers.