Preheat oven to 350°. Butter 13x9x2 inch baking pan; set aside. In large mixing bowl stir together 2 cups flour, granulated sugar, baking powder, salt, soda, allspice and cloves. Make well in center of dry ingredients; set aside.
In small bowl combine oil, milk and eggs. Add to dry ingredients and stir until moistened. Stir in rhubarb. Spread in prepared pan. Set aside.
In food processor bowl combine 2/3 cup flour, brown sugar, cinnamon and butter. Cover and process with several on-off turns until mixture resembles coarse crumbs. (Or, in medium bowl stir together flour, brown sugar and cinnamon. Using pastry blender, cut in butter until mixture resembles coarse crumbs.) Stir in nuts, if desired.
Sprinkle evenly over batter. Bake about 35 minutes or until wooden toothpick inserted in center comes out clean. Serve warm or cool completely on wire rack. Top with whipped cream, if desired.