Before baking, preheat oven to 350°F / 180°C / Gas Mark 4
One day ahead: There will be approximately 6 to 7 pounds of meat. Slash the meats in several places down to the bone and rub the salt well into it. Set aside.
Heat the comal or griddle and toast the ancho, cascabel and guajillo chilies lightly, turning them from time to time so that they will not burn. Remove the veins and seeds. Put the chilies to soak in hot water for about 20 minutes.
Drain the chilies and transfer them to a blender jar. Add peppercorns, cloves, oregano, cumin seeds, vinegar, garlic, onion and blend to a smooth sauce. Cover the meat thickly with the paste and set it aside to marinade for about 18 hours.
On serving day: Preheat the oven to 350°F. Put the water in the bottom of a large Dutch oven or casserole with a tightly fitting lid and place the meat on a rack so the it is just about the water. Seal the lid with a paste of 3 parts flour and 1 part water, and cook for about 3 1/2 to 4 hours. At this point the meat will be falling off the bones. Shred the meat and remove the bones.
Strain off the juices from the bottom of the pan, cool, and skim off the fat. There should be about 2 cups of juices, if not, make it up with up to 2 cups with water. Blend the tomatoes to a smooth sauce. Put the sauce and the skimmed juices from the meat into a saucepan and bring to a boil.
Serve each portion of mixed meats in a deep bowl topped with the sauce. Eat with tortillas.