Preheat oven to 350F. Grease and flour a 9x5-inch baking dish. Set aside. In a medium bowl, whisk together flour, sugar, baking soda, salt, and spices. Set aside. In a large bowl, stir together buttermilk, pumpkin puree, and oil until combined. Stir in eggs and vanilla until incorporated. Add dry ingredients, and stir until just combined. Stir in nuts if using. Pour batter into prepared pan. Bake for 55-65 minutes, or until a wooden toothpick inserted in the center comes out clean. Remove from oven, and let cool in pan for 10 minutes. Remove from pan and finish cooling on wire rack.