I would be careful adding weight behind the back axle. Have you ever towed a trailer or seen a trailer with too much tongue weight? It uses the back axle as a fulcrum and takes weight off the front tires. This leaves you in a situation when you do a hard brake, the front tires lock up and slide because they have no weight.
Just my 2 cents, but if I were to add weight, I would place it over, or slightly in front of the rear axle.
Jon