Place cake mix into a large mixing bowl. Beat in pumpkin, 3/4 cup brown sugar, vegetable oil, cinnamon, allspice, and nutmeg with an electric mixer until cake mix is moistened, about 1 minute. Beat in eggs; continue beating batter for 2 more minutes.
Pour batter into prepared tube pan.
Melt butter with 1/2 cup brown sugar in a saucepan over low heat until brown sugar has dissolved; stir in pecans. Spoon half the pecan mixture over the cake batter.
Bake cake in the preheated oven until lightly browned at the edges and a toothpick inserted into the center of the cake comes out clean, about 1 hour.
Cool cake in pan for 15 to 20 minutes, then invert cake onto serving platter. Allow cake to cool completely before drizzling the remaining half of the pecan topping over the cake.
