Spread the diced pork on a non-reactive sheet pan and sprinkle with kosher salt. Cover with plastic and place in the refrigerator.
Hold the dried, cleaned Ancho and Guajillo chilies with a pair of tongs over an open flame until toasted. Place in a small saucepan and add 2 cups of the beef stock.
Heat a skillet over high heat and add the Japones chiles. Stir and toss over high heat until well toasted. Remove from skillet and add to saucepan. Bring the saucepan to a boil over high heat and then drop to a low simmer. Cook the chiles until very tender. Take off the heat and add the Piloncillo. Stir to dissolve. Pour all contents into a high-speed blender and purée until smooth. Set aside.
Place a large cast-iron Dutch oven on the stove over medium heat. Add the bacon and cook until the bacon is brown and the fat is rendered. Add the yellow onion and garlic and cook until tender.
Pour the Masa Harina over the diced pork and toss until well coated. Add the pork to the Dutch oven and brown, being careful to scrape the bottom of the pot each time you stir. Add the diced tomatoes and tomato sauce and stir until smooth. Add the remaining two cups of the beef stock and stir until smooth, continuing to scrape the bottom of the pot. Bring to a boil and simmer slowly until the pork is fork tender, about 1 to 1 1/2 hours. Add kosher salt and fresh ground pepper to taste.