To prepare crust, combine crushed crackers, 1/4 cup margarine, and granulated sugar in a mixing bowl. Press mixture firmly in a 9" pie pan. Chill one hour, or until firm. Preheat oven to 425. In a mixing bowl, combine pumpkin, sweetened condensed milk, egg, two teaspoons cinnamon, ginger, nutmeg, and salt. Mix well. Pour into crust. Bake for 15 minutes. Meanwhile, in a smaller bowl, combine brown sugar, flour, and remaining cinnamon. Cut in margarine until crumbly. Stir in walnuts. Sprinkle mixture over top pie. Reduce oven to 350. Bake for 40 more minutes, or until set.