Spread with mustard; sprinkle with chili powder. Cover pan with foil; place on grate of grill. Cover grill with lid.
Grill 3 to 3-1/4 hours or until meat is done (145F).
Remove meat from grill; let stand 10 min. Meanwhile, mix barbecue sauce, tomato sauce, molasses and garlic in large saucepan. Reserve 1 cup barbecue sauce mixture for later use.
Remove meat from pan; shred with 2 forks.
Add to barbecue sauce mixture in saucepan; cook on medium-low heat 10 min. or until heated through, stirring occasionally.
Serve in buns with the reserved sauce on the side.