In a small bowl, mix together salt, paprika, onion powder, thyme, white pepper, black pepper, cayenne pepper, and garlic powder. Remove and discard giblets from chicken. Rinse chicken cavity, and pat dry with paper towel. Rub each chicken inside and out with spice mixture. Place 1 onion into the cavity of each chicken. Place chickens in a resealable bag or double wrap with plastic wrap. Refrigerate overnight, or at least 4 to 6 hours.
Preheat oven to 250 degrees F (120 degrees C).
Place chickens in a roasting pan. Bake uncovered for 5 hours, to a minimum internal temperature of 180 degrees F (85 degrees C). Let the chickens stand for 10 minutes before carving.