Hi, put the tank in the back shakespears have a short bow and with two people up front thats more than enough weight up there. What shakespear is it, Waverider Magnum? Is it exactly 16' or is it slightly smaller?
A friend of mine had one with a v4 johnson on it 115 or 135 I can't remember which but it fairly shifted. What engine do you have now?
Heres a link to a good site it has all the models and some owner submitted info on engines and speed.
It says 56 knots with 115hp in a 16'er, not bad!!!