Combine ¼ cup melted butter and ¾ cup brown sugar in a 9x13 baking dish and spread evenly over the bottom of the dish.Lay bread slices over the brown sugar.Whisk together eggs, half and half, vanilla, cinnamon, ginger, nutmeg, Grand Marnier, and salt until well blended.Pour egg mixture over the bread slices, covering each piece of bread with liquid.Cover and refrigerate overnight or for several hours.Remove from refrigerator at least half an hour before baking.Preheat oven to 350.In a large mixing bowl, stir together flour, brown sugar, cinnamon, nutmeg, and salt until well mixed.Using your fingers, work the butter in until the mixture resembles large crumbs.If using nuts, stir in the pecans and mix well.Do not over mix or your topping will turn mushy.Sprinkle all of the topping evenly over the casserole prior to baking.Bake casserole until set and top is nicely browned, about 45 minutes. Remove and let cool slightly before serving.