In a mixing bowl, combine flour, granulated sugar, and brown sugar; cut in butter with a pastry blender. Stir in chopped pecans. Reserve 1 cup crumb mixture; set aside. Press remaining crumb mixture over the bottom and about halfway up the sides of a 12x7x2-inch baking dish.
In a mixing bowl, combine pumpkin, sweetened condensed milk, eggs, cinnamon, allspice, and salt. Mix until well blended. Pour pumpkin mixture over the crust. Top with remaining 1 cup crumb mixture. Bake at 350° for 50 to 60 minutes, until browned and set. Cool. Cut into bars and serve with ice cream or a dollop of whipped cream or whipped topping. Makes about 8 servings. Keep pumpkin bars refrigerated.