Preheat oven to 350 degrees F. Grease and flour a 15x10x1-inch baking pan; set aside. In a large mixing bowl beat cake mix, pumpkin, eggs, water, spice, and baking soda with an electric mixer on low speed until moistened. Beat on medium speed for 3 minutes. Pour batter into prepared pan.
Bake for 10 to 12 minutes or until a wooden toothpick inserted near the center comes out clean. Cool in pan on a wire rack 10 minutes. Remove from pan and turn out onto a clean kitchen towel sprinkled with powdered sugar; cool completely.
Cut cake crosswise to make 3 10x5-inch rectangles. To assemble, place one rectangle on a cake platter. Tint frosting with orange paste. Spread one-third (about 1/2 cup) of frosting over cake rectangle. Place a second rectangle atop; frost with another third of frosting. Top with last cake rectangle; spread with remaining frosting.
To decorate top, place 3 tablespoons of melted coating in a sealable plastic bag. Cut a small opening in one corner of the bag. Seal well, and drizzle coating across the top and down sides of cake. To make pumpkins and ghosts, divide remaining coating in half. Add a small amount of orange food coloring paste to half for pumpkins. Leave other half plain. (If coating firms up when paste is added, add 1 teaspoon shortening; heat mixture over low heat until melted.)
Line a baking sheet with parchment or waxed paper. Separately spread melted coatings to about 1/8-inch thickness. Place in freezer for 1 to 2 minutes or until coating is firm. Cut out desired shapes. Place atop cake. Makes 12 servings.