Preheat the oven to 350°F., To make the cake: Grease and flour a large rimmed half-sheet pan, about 18" x 13"; or line the pan with greased parchment. Note: to make these bars in a 9" x 13" pan, see "tips," below., Beat the oil and the sugars together until well blended., Beat in the eggs, then the pumpkin purée., Stir the baking soda, baking powder, salt, and spice into the wet ingredients, then stir in the flour, beating gently to combine., Stir in the nuts and raisins/ginger., Pour the batter into the prepared pan, and bake for approximately 20 to 25 minutes, or until a cake tester or toothpick inserted into the middle of the cake comes out clean., Cool the cake in the pan for 10 minutes, then turn it onto a rack to finish cooling; or leave it right in the pan, if you've lined the pan with parchment., To make the frosting: Beat the cream cheese and butter together., Beat in the maple syrup, then mix in the confectioners' sugar and salt. Adjust the consistency of the frosting with additional maple syrup or confectioners' sugar, if necessary; it should be nicely spreadable., Frost the pumpkin cake bars and sprinkle with the toasted walnuts. Once the frosting has set somewhat, cut into squares or rectangles.