The strut I posted the picture of above is mounted through 4.5mm of plywood, 1.5mm of bottom skin and a 3mm doubler. Since you're using a flex cable, I would install the strut parallel to the bottom of the boat initially. You also don't want to mount the stuffing tube straight. You want to give it a slight "S" curve so that the cable doesn't "whip" inside of the tube. Line up the inner end so that the cable goes into the center of the tube, give it a gentle downward bend to take it through the boat bottom and then another gentle upward bend to line up with the strut. You mentioned needing a prop to know how long to cut the cable. Actually, you need a thin walled drive dog instead. This is what the prop locks into to get it to spin. When you cut the cable, be sure to cut it long enough to allow one cable thickness between the front of the drive dog and the rear of the strut to allow for shrinkage when the prop is loaded or you will burn up your motor, strut or other parts due to excess load. As for strut depth, line up the strut tube center with the bottom of the sponson transoms. This is normally a good starting point but, unfortunately, I don't have a recommendation for you as far as what prop to use.