Measure out 1/2 cup of cake mix and set aside. In a small bowl, combine the remaining cake mix, oil, and 1 egg. Press into a greased 9x13 baking pan. Sprinkle with 1/2 cup walnuts. In a large bowl, mix the pumpkin, reserved cake mix, milk, vanilla, salt, pumpkin pie spice, and remaining eggs. Pour over crust, then sprinkle with remaining walnuts. Bake at 350 degrees for 45 - 50 minutes or until a knife inserted in the center comes out clean. Cool pan on a wire rack. Cut into squares.