Preheat the oven to 350 degrees F. In a large bowl, combine the flour and baking soda. Add the margarine, brown sugar, walnuts, and vanilla. Blend well. Press the mixture, a small amount at a time, into a 9-inch pie plate, building up the sides and fluting the edge. Cover the fluting with a strip of aluminum foil to prevent overbrowning.
In another bowl, thoroughly combine the cornstarch, cinnamon, ginger, nutmeg, and granulated sugar. Add the pumpkin, cream, eggs, and molasses. Pour into the crust and bake for 1 to 1-½ hours, or until the center is set.