Re: 18 ft repower
As long as you have the space in the boat, and the outdrive can handle the additional power/torque, then it can be done. It just matters if the difference in weight is exponentially more than the 4cyl set up.
If you find that there are identical boats with 8's in them, then it was just a matter of what the manufacturer of your boat was trying to accomplish with a performance/price point.