Green tripe has the advantage of adding weight (and energy for working
dogs), but it is also low
in phosphorus and protein, so great for kidney failure
dogs. The drawback is it stinks to high hell when it's cooking, but dogs generally absolutely love it.
Other good foods for adding weight are fatty meats - so pure hamburger (no additives, herbs or onions), lamb and beef. Other things are sweet potatoes, but sparingly, as they can cause loose stools.