Butter an 8 to 10-cup bundt pan. Dust the pan with fine, dry bread crumbs and tap out excess. Sift together flour, baking powder, baking soda, and salt. Set aside. In large mixer bowl, beat the butter until soft. Add sugar, beating well. Add the eggs, one at a time, beating well after each. On low speed, add the dry ingredients alternately with buttermilk, beating only until smooth after each addition. Stir in the rinds and then the nuts. Turn into prepared pan and smooth the top. Bake at 350, on a rack 1/3 up from the bottom of the oven, for 55 to 60 minutes or until the top springs back sharply when it is lightly pressed with a fingertip.
Meanwhile, prepare the glaze. Place both juices and sugar in a small saucepan over moderate or high heat. Stir with a wooden spatula until sugar dissolves and mixture comes to a low boil. Remove from heat and stir in the rum.
When cake is done, set it on a rack to cool. Pierce all over the top with a cake tester or toothpick. Spoon the hot glaze over the cake about a Tbsp at a time. When about half the glaze has been added, use a small spatula or knife to ease the sides of the cake away from the pan so that glaze can run downward. Continue adding the glaze and releasing the sides until all the glaze has been absorbed. Let stand for about 10 minutes, until the bottom of the pan has cooled. Then invert onto plate. Bang plate & pan against work surface if necessary to release cake. Let stand for a few hours, then cover with plastic wrap. Let stand overnight. Cut into thick slices.