You really need to provide some info on the boat and its rating as found on the capacity plate. In a perfect world, you could do some things to improve the trim on your boat. However, in the real world, unless your boat is a wooden stitch-and-glue craft, there isn't much you can do that will have an effect. Even then, you would have to go through some serious rebuilding to get what you want-- a wider stern.

Another down side is that by adding weight, such as the 300 pounds of sand bags in the bow, you are affecting the handling and freeboard of the boat. My hunch is that you'll find you've exceeded the weight capacity of your boat. That is a dangerous situation.