Mix flour, brown sugar, margarine, coconut, and pecans, if desired. Place in 9x13 pan. Bake at 350 degrees for 15 minutes. Stir. Bake another 15 minutes or until golden. Cool. Set aside half. Cover bottom of pan with remaining half. Prepare 2 packages of instant pudding with milk. Fold in whipped topping. Pour over crumb mixture. Cover pudding with remaining crumb mixture. Chill and serve.