Heat oven to 350 degrees F. Generously season pork with salt and pepper; place in shallow roasting pan; roast for about 2 1/2 hours (about 20 minutes per pound), until internal temperature on a thermometer reads 145 degrees F. Remove roast from oven; let rest about 10 minutes before slicing to serve with stuffing.
Meanwhile, in large bowl thoroughly combine ground pork, bread cubes, broth, onion, celery, walnuts and seasonings; mix well. In medium saucepan, combine rhubarb and sugar; bring to a boil. Pour over stuffing mixture; mix lightly. Spoon into buttered 2-quart casserole. Cover, bake at 350 degrees F. for 1 1/2 hours.