PREHEAT grill to medium heat. Place pork in disposable roasting pan. Spread mustard over pork; sprinkle with chili powder. Cover pan with foil; place on grate of grill. Cover grill with lid.
GRILL 3 to 3-1/2 hours or until pork is cooked through (160°F). Remove pan from grill; let stand 10 min. Meanwhile, combine barbecue sauce, tomato sauce, molasses and garlic in large saucepan. Remove 1 cup of the barbecue sauce mixture; set aside for later use.
REMOVE pork from pan; shred meat using two forks. Add to barbecue sauce mixture in saucepan; cook on medium-low heat 5 min. or until heated through, stirring occasionally. Serve in buns with the reserved barbecue sauce mixture on the side.