In a small bowl, mix together salt, paprika, onion powder, thyme, black pepper, cayenne pepper, and garlic powder. Remove and discard giblets from chicken. Pat dry with paper towels, then rub each chicken inside and out with spice mixture. Mince the garlic cloves with a garlic press, then rub them into the chicken cavities and 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.
You can make a gravy with the drippings, or baste the chicken with the drippings for a nice glisten. If you have leftovers, you can use them for enchiladas and the bones can be used in soup.
