In a medium mixing bowl, whisk together both sugars, cinnamon, salt, graham cracker crumbs and flour. Add melted butter (cooled!) and stir everything together with a fork until coarse crumbs form.Preheat oven to 350 degrees F.Grease a 13" x 9" baking dish.Arrange bread cubes in the dish.In a large mixing bowl, whisk together milk, eggs, pumpkin puree, vanilla, pumpkin pie spice and sugar.Pour pumpkin mixture over bread cubes. Gently toss to coat. If you are preparing this dish the night before, cover the dish at this point with saran wrap and refrigerate overnight. In the morning, let the dish sit on the counter for 15 minutes, to come to room temperature.Sprinkle with graham cracker crumb topping.Bake 38 to 42 minutes, until top is crunchy and egg mixture is set.Dust with powdered sugar, if desired.Serve with maple syrup, if desired.