Hi, I'm going to be towing a small boat soon, and am new at this. I'm getting a hitch installed in the next few days (Draw-tite 76112, 2" receiber), and I asked the foremost expert on towing this boat with this exact trailer (the boat manufacturer) how high the ball should be. He said the top of the ball height should be 18". What parts to do I need in order to end up with the ball at that height on my 2011 Sienna LE?