Put oven rack in upper third of oven and preheat oven to 400°F. Scatter bread in a single layer in 2 large shallow baking pans (17 by 12 inches) and toast, stirring once or twice and switching position of pans halfway through baking, until golden and dry, about 15 minutes. Transfer to a very large bowl. Cook pancetta in a 12-inch heavy skillet over moderate heat, stirring occasionally, until browned, 12 to 15 minutes. Add butter and heat until melted, then add celery and onions and cook, stirring occasionally, until softened, about 12 minutes. Stir in sage, salt, and pepper and cook 1 minute. Add pancetta mixture along with chestnuts and prunes to bowl containing bread. Whisk together stock and eggs, then stir into bread mixture until combined well. Transfer to baking dish (stuffing will mound above dish). Bake, loosely covered with a buttered sheet of foil (buttered side down) 30 minutes, then remove foil and bake until top is browned, 10 to 15 minutes more.