the correct motor position, is relevent to the length of the shaft, and the angle, and the point at which it exits the hull
many of the old plans knocking around are for ic set up, which itself requires differences to electric set up, and again twin or single screw. There have been over the last two years a number of really nice Huntsman builds, that have failed at the last hurdle, because the builder has tried to remain loyal to the original ic plans, and the results have been an underpowered (due to the prop angle being too great) or really poor handling, as the rudder has been far too cllose to the end of the hull, or too far down the hull.
On the fairey owner website, there is some very good reference to the prop angle, and rudder position
Here you can see the prop is not in the centre of the hull, and the attitude of the propellers will propel the boat forwards, if the attitude was more of an angle, then the boat would lose forward propulsion as its driving the boat upwards (in theory!), this can then make the rear of the hull "light" and attribute to poor handling, particularly cornering at speed, something a number of builders have complained about in the past. Let me say these hulls handle beautifully, the more abuse they get, the better they go
you could always try this novel idea of rudder angle, this was a recent visit to Newsham Park, Liverpool, in the centre of the lake! Goodness knows what I hit, it also broke a blade off the prop and moved the shaft, luckily didnt sink