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My 19 Outrage II (same hull as your Outrage 18, but different interior configuration) is fitted with a 6" set-back bracket with a Yamaha 150 2-stroke (430 lbs plus about 25 lbs for oil tank reservoir in stern locker). It was rigged like this when I purchased it, so I can't give you any insight into "before and after."

It seems to work well enough as configured. The Detwiler bracket allows the engine height to be more finely adjusted, since you aren't limited to mounting-hole increments (it is a sliding track system.) Mine seems to be raised up pretty darn high but I have never fiddled with it.

My impression is that the boat squats a fair bit with the engine hanging so far aft, unsupported by the buoyancy of the hull. So my suggstions would be to go with a 4" set-back bracket, to limit stern squat.

If you find line drawings that show the fore-and-aft center of buoyancy, please let us know.