Yes replace Nasty Plot what good is Nasty Plot if you're giving Lucario a Choice Specs.
Personally I wouldn't give Lucario Hidden Power Grass but it's your decision. Since your running a Special set I would give Lucario Flash Cannon or Psychic. Flash Cannon is the preferred choice since Lucario is a Steel type, it gets STAB, has a 10% chance of lowering the target's Special Defense and is able to hit Fairy types. Although Psychic is also a good move as it hits other Fighting types and also has a 10% chance of lowering the targets Special Defense.
If you want to keep Nasty Plot get rid of your Choice Specs and give Lucario either the Lucarionite (if you playing in Ubers), a Life Orb or Leftovers.
Hope I helped :)
