You can use stick-on wheel weights. Cheap and easy. You can even stick them around the upper hull if things get tight. Can be stacked to centralize. I have some in the front of my T34 and Pershings to gave a C-of-G around the middle, all turn and drive over stuff much better then stock.
example:http://www.amazon.com/Adhesive-Truck...69DYJ5F4CH8S3E


....beat me to it, Imex-Erik