Preheat oven to 325. Prepare a 9"x13' pan with cooking spray; set aside. To prepare topping, combine brown sugar, flour, and walnuts. Cut in margarine until mixture is crumbly; set aside. To prepare cake, combine cake mix, pumpkin pie spice, and baking soda in a mixing bowl; set aside. In another mixing bowl, combine pumpkin, egg whites, and water. Add to dry ingredients and mix until all is moistened. Spread batter into prepared pan. Sprinkle with topping mixture. Bake for one hour or until toothpick inserted into center comes out clean.