Sylvester Stallone put on 40 pounds to play Nowheresville, N.J., sheriff Freddy Heflin in Cop Land, but the big deal isn't his gut -- it's his acting. There's no flab in his portrayal of a deaf, dull-witted lawman -- Freddy is a sweet slug -- who rediscovers his moral conscience. Branching out in a bold new direction, Stallone is quietly devastating. James Mangold has directed Cop Land from his own ardent, audacious script, and despite some draggy, overdeliberate moments, it's the strongest…