Place meat in a large, heavy plastic bag. In a small bowl combine celery salt, 2 cloves of garlic, pepper, liquid smoke and Worcestershiresauce. Pour over meat and seal bag. Refrigerate for 24 hours, turning bag over several times.
Remove meat and place on a piece of heavy duty foil. Pour marinade over meat and seal foil. Place in a baking pan. Bake in a 300 degree oven for 5 hours.
Meanwhile, remove and discard stems and seeds from chilis. Place chilis in abowl and pour boiling water over them; let stand until soft (about twenty minutes). In a blender whirl chilis and about half the soaking liquid (reserve remainder) to make a thick puree. Set aside.
In a 2 qt sauce pan, heat oil over medium heat; add flour and stir until lightly browned. Stir in remaining 2 cloves garlic, cumin, and oregano. Place a wire strainer over pan. Add pureed chilis, forcing them through with the back of a spoon; add remaining soaking liquid to help force puree through. Cook, stirring, until bubbly. Season to taste with salt.
Remove brisket fromoven. Carefully drain liquid; reserve and refrigerate. Spoon chili mixture over meat; top with onion. Reseal foil;continue to bake until meat is tender when pierced (1 more hour).
Lift brisket from foil and cut into 1/4 inch thick slices (you can shred the meat if you prefer). Drain chili sauce into a pan. Discard solidified fat from chilled marinade; add marinade to chili sauce and heat over medium heat.
Serve meat and sauce in tortillas. Top with lettuce, cheese, sour cream, onion, etc. as desired.