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