8 tablespoons (1 stick) unsalted butter, plus more for dish
6 slices good white bread, crusts removed, torn or diced into 1/4- to 1/2-inch pieces
5 1/2 cups milk
1/2 cup all-purpose flour
2 teaspoons salt
1/4 teaspoon freshly grated nutmeg
1/4 teaspoon freshly ground black pepper
1/4 teaspoon cayenne pepper, or to taste
4 1/2 cups grated sharp white cheddar cheese (about 18 ounces)
2 cups grated Gruyere cheese (about 8 ounces) or 1 1/4 cups grated Pecorino Romano cheese (about 5 ounces)
1 pound elbow macaroni (or other small pasta shape)