Preheat oven to 325°F.
Butter two 9x5x3-inch loaf pans. Coat pans with flour, tapping out excess.
Combine first 7 ingredients in medium bowl; stir to blend well.
Whisk eggs in a large bowl. Add sugars and buttermilk, mixing well. Add pumpkin puree and oil; whisk until blended.
Add bran cereal to pumpkin mixture, stirring well to combine and allow to soak for 15 minutes.
Add flour mixture to pumpkin mixture; stir to combine. Fold in nuts and raisins.
Divide batter evenly between prepared pans.
Bake until tester inserted into center of each loaf comes out clean, about 1 hour 20 minutes.
Allow loaves to cool in pans for 10 minutes. Use a knife along pan sides to loosen loaves, if necessary.
Turn out loaves onto racks and allow to cool completely.