I'm afraid the boat is just too small... you have enough power to easily push a 40+ inch boat around, and still have serious speed. These projects are fun, and they will show you the practical limits of power. You did a great job, I just don't want to see you waste a lot of time and effort if it's not going to do what you want it to. I would look on ebay maybe for a deepvee hull that included controls & linkages, might be able to find one pretty cheap. I know it might sorta defeat the purpose of what you are trying to do, but if you want a performer.....
And putting the prop behind the rudder will make for even more weirdness. You can try it... but I don't think it'll steer at all
