Preheat oven to 350°. Place roast on a rack in a large shallow roasting pan. Sprinkle with 1/4 teaspoon pepper. Cover rib ends with foil. Bake, uncovered, for 1-1/2 hours.
Melt butter in a Dutch oven over medium heat. Cook and stir celery and onion until tender, 3-5 minutes. Stir in the stuffing, corn, mushrooms, salt, poultry seasoning and remaining pepper. Carefully spoon 1-3 cups into center of roast. Place remaining stuffing in a greased 2-qt. baking dish. Refrigerate until ready to use.
Bake roast until a thermometer inserted in stuffing reads 140°, 1 hour. Cover and bake extra stuffing until browned, 30-40 minutes. Transfer to a serving platter. Let stand for 15 minutes. Remove foil. Cut between ribs to serve.