Divide the bread slices between two 9- by 13-inch baking pans, and arrange in a single layer. In a medium bowl whisk together the milk, the eggs, the orange juice, the sugar, the salt, the vanilla, and the Grand Marnier. Pour the mixture over the bread, turning to coat evenly. Cover with plastic wrap and refrigerate for at least 2 hours. Heat 2 teaspoons of the butter in a large nonstick skillet over medium heat. Add the bread, 2 to 4 slices at a time, without crowding, and cook until golden brown on both sides, 2 to 3 minutes per side. Remove the bread from the pan, place in a single layer on a baking sheet, and place in a preheated 350`F oven to keep warm. Repeat until all of the slices are browned, adding more butter for each batch. Serve with the maple syrup.