Preheat oven to 425 degrees F. In a large bowl, mix together pumpkin, sweetened condensed milk, and egg yolks. Stir in cinnamon, ginger, nutmeg, and salt. In a large bowl, whip egg whites until soft peaks form. Gently fold into pumpkin mixture. Pour filling into pie shell. Bake for 15 minutes. While the pie is baking, prepare the topping: In a small bowl, combine flour, brown sugar, and cinnamon. Blend in cold butter with a fork or pastry blender until the mixture is crumbly. Mix in chopped nuts. Sprinkle topping over pie, then reduce heat to 350 degrees F. Bake an additional 40 minutes or until set. Wrap foil around the crust if it begins to brown too quickly.