Preheat oven to 350 degrees F. Place the roast in a roasting pan. Sprinkle and rub with salt, pepper, chili powder and cumin. Roast the beef to the desired doneness; about 70 to 75 minutes should cook it rare to medium rare. Set the roast on a plate, tent with foil and rest 10 minutes.
While the meat rests, set the roasting pan on the stovetop over medium-high heat. Add the barbecue sauce and stock, bring to a simmer, and then reserve on low heat.
After it has rested, very thinly slice the beef and pile it on the bottom half of the buns. Drizzle the meat with some of the sauce mixture from the pan. Set on the lettuce, pickles and top buns and serve.
Note: Mini hamburger buns are sold in the bakery department. Supply can sometimes be limited, so you may wish to order them in advance.