Let's see. My Conkeldurr is Drain Punch, Mach Punch, Bulk Up, and Stone Edge.
I would replace Hammer Arm for Drain punch, to recover off that burn damage. Since Rest leaves you invulnerable for 3 turns, that's plenty of time to give your opponent to set up, or just give you back the damage you recovered. Drain Punch is more reliable, taking out common pokemon such as Blissey, Tyranitar, Scrafty, and others. Plus, who doesn't want more health?
With base 65 Sp.Def, those 128 EVs and Calm Mind isn't going to help you much. I don't think he can even learn Calm Mind. I say put those EV's for more HP and Defense. Replace Calm Mind for Bulk Up or Facade. Facade will help with that attack boost from Guts, and it doubles in power after the status ailment. Go with Bulk Up if you want him to be a physical tank.
Mach Punch is good, gives him priority and potential revenge killing.
Payback is a good move. Doubled in power when used AFTER the opponent.It works well with his speed. It covers his Weaknesses, and can hit ghosts and Psychics. However, against a Psychic type pokemon, this move is okay. Psychics are usually faster than Conkeldurr, but a Psychic will kill you off. Against Ghosts, it's an okay move. If you can survive a Shadow Ball, then it's an okay Move choice. However, his sub par special Defense will not be kind. Stone Edge will cover your flying weakness, plus it hits Fire, Bug, and Flying type pokes hard. It can also hit ghosts and Psychics. Stone Edge or Rock Slide is a better idea, unless your SURE you will survive a Shadow Ball or a Psychic (which is unlikely).