Melt the butter in a skillet and stir in the onion and celery. Cook over low heat until the onion is soft, add this mixture to the crumbs and toss lightly with plenty of pepper and salt.
Preheat the oven to 350 F. Rub the pig with the oil and sprinkle with salt and pepper. Stuff lightly with the stuffing and sew the cavity shut. Put the pig on its side on a rack in a shallow roasting pan. Cover the ears and tail with foil so they do not burn. Place in the oven and baste every 20 minutes with the melted butter until there are enough pan juices for basting. Roast about 2 1/2 hours, basting often, then remove foil. Roast about 30 minutes more, until the internal temperature is 165 F. Remove and let rest at room temperature for 15 minutes before carving.