a few things, first i always make sure that the engine is high enough that when the prop is as far down as it will go that it clears the back edge of the hull slightly that way no thrust is wasted and blowing into the hull and comming back forward(dont think its causeing the prop though) i doubt that the engine head being over makes a whole lot of difference either, is it strait with the hull, also you can turn it one way or another to counter prop walk, hows it point vertical wise, if its lifting the back to much it could be causing a spin id imagine. also higher pitch equals more prop walk, find something that the engine likes and go from there, this is also why you had less thrust it just didnt have the stuff to turn that bigger prop, i have a 16-8 on my boat but its smaller than your engine a little i believe maybe an 18-10 like you had or and 18-8 to help the walking. hope that helps a little, mine needs a bigger motor or smaller hull as its 40-20 and to much for a motor that swings a 16 incher, but it runs great on water(id like nore speed though) there are picks of it under a thread titled something like pics of my bigboat, i like it for water and snow though but it wil be upgraded when i get a chance(twaeking in my nitro vee at the moment)....