Combine squash, milk, sugar, 1/3 cup brown sugar, eggs, cinnamon, nutmeg, and salt in a large mixing bowl; beat well. Pour mixture into pastry shell. Bake at 375° for 20 minutes. Remove from oven.
Combine pecans, 1/2 cup sugar, and butter; mix well. Sprinkle mixture over top of pie. Bake at 375° an additional 25 minutes. Cool slightly, and serve with whipped cream.