Shredded Beef in Guajillo Sauce (Carne Deshebrada en Salsa Roja )

Put all beef ingredients in a 5- to 6-quart heavy pot with 1 teaspoon salt and enough water (about 2 quarts) to cover beef. Simmer, partially covered, skimming foam and adding more hot water if necessary to keep beef covered, until tender, about 2 hours.
Remove beef and shred (once cool).
Strain broth through a fine-mesh sieve into a bowl, discarding solids, and skim off fat.
Preheat broiler.
Toss tomatoes with 1 tablespoons oil in a 4-sided sheet pan. Broil 3 to 4 inches from heat until softened and charred, 10 to 12 minutes.
Slit guajillo chiles lengthwise, then stem and seed. Heat a dry heavy medium skillet (not nonstick) over medium heat until hot, then toast chiles in 2 batches, opened flat, turning and pressing with tongs, until more pliable and slightly changed in color, about 30 seconds per batch. Transfer to a bowl and soak in hot water to cover until softened, 20 to 30 minutes.
Toast peppercorns, cumin seeds, allspice, and clove in hot skillet over medium heat, stirring constantly, until fragrant and a shade darker, 30 seconds to 1 minute. Transfer to a blender.
Drain soaked chiles and purée in blender with toasted spices, broiled tomatoes, onion, garlic, 3 chiles de árbol, 2 cups broth, and 1 teaspoon salt until smooth, about 2 minutes. For a spicier sauce, add remaining 2 chiles de árbol and purée again.
Heat remaining 2 tablespoons oil in cleaned pot over medium-high heat until it shimmers, then slowly add sauce, stirring (it will spatter), and simmer, stirring frequently, 5 minutes. Add beef and 1 cup broth (reserve remainder for another use) and cook over medium-low heat, stirring, until meat is heated through, about 5 minutes.