Search this side as well as the internet for proper outboard height. If you and a friend went out on the boat and got it up to cruise speed, or faster, even as fast as wide open throttle, go back and look at the outboard and see where the antiventilation plate is running. It's the flat surface immediately above the prop. It should be skimming right on the surface of the water. If it's buried in the water while running at that speed, then the outboard needs to be raised up.
Taking a photo while running that way will help.
What prop, size, pitch and diameter does it have? Is it aluminum or stainless? How many rpm's are you getting at wide open throttle and what is the top speed?
I may be able to help knowing those things.
Martin