The four strokes are heavy. If moving the fuel and batteries to the bough is not enough, I doubt that adding ballast with sandbags will do anything to improve the safety or performance of the boat. If two guys in the stern sinks the boat, you should probably replace either the hull or the motor. I bet this is not what you want to hear but you will be better off with a well balanced boat than some kind of added flotation or other bandaid type solution. I would think long and hard before modifying the hull.