Butter a 11/2 quart oblong baking dish. Sprinkle with half of the ginger snap crumbs. In a medium bowl, stir pumpkin, sugar, 1/2 cup nuts, pumpkin pie spice, and salt until well blended. In a large bowl, stir ice cream until softened. Stir in pumpkin mixture until well blended but not melted. Evenly pour into baking dish. Sprinkle with remaining crumbs. Cover dish with foil. Freeze for 3 hours or until firm. To serve, cut with a sharp knife. Garnish with whipped cream and nuts.