Pour boiling water over prunes. Let stand 15 minutes. Drain, pit, and dice. Add lemon rind, set aside. Grease and flour 9" tube pan. Sift together flour, baking powder, baking soda, salt. Remove 1/4 cup of flour mixture and toss with prunes. Cream butter and sugar until fluffy. Beat in eggs, one at a time. Slowly beat in flower mixture, alternately with sour cream and vanilla, beginning and ending with flour. Fold in prunes. Combine brown sugar, cinnamon and nuts. Turn 1/3 of batter into pan. Sprinkle 1/3 brown sugar mixture. Repeat 10 and 11 two more times. Bake at 350 deg F for 55 minutes or until done. Cool in pan on rack 10 minutes and remove from pan.