The best item would probably be a Choice Band or a Life Orb, Choice Band increases attack by 50% but locks you into one move, but since you'll mainly be using Mach Punch because it has priority +1, it is quite clear to use Choice Band, but I only recommend Choice Band for Generation 6 and below, because UU threats like Mimikyu are sure to KO you as you are locked into Mach Punch, Life Orb gives more options but takes away ⅒ of your HP, but worth it. Heavy-duty boots is also suggested as it allows Infernape not to take hazard condition, for the moveset, even boosted with Iron Fist, Flare Blitz is still much better than Fire Punch, and a good trap is to use Infernape's Speed and U-Turn, I recommend changing Thunder Punch with U-turn, but Iron Fist would still put in work as I said in the beginning of the answer:"You'll be mainly using Mach Punch"
If you use a life Orb, here are the results:
252 Atk Life Orb Infernape Shadow Claw vs. 0 HP / 0 Def Mimikyu: 205-244 (81.6 - 97.2%) -- guaranteed 2HKO
But with Choice Band, it would be different:
252 Atk Choice Band Infernape Shadow Claw vs. 0 HP / 0 Def Mimikyu: 238-280 (94.8 - 111.5%) -- 68.8% chance to OHKO
So it literally doesn't make much difference since both are around 2HKO, but using Choice Band is risky. So overall, I would pick a Life Orb over Choice Band and Heavy-duty boots. Although I recommend having a Defogger or Rapid Spinner on your team.
EDIT: Heavy-duty boots would not help as actually Infernape is bot weak to Stealth Rocks
Here is a recommended set:
Infernape @ Life Orb
Ability: Iron Fist
EVs: 252 Atk / 4 SpD / 252 Spe
- Mach Punch/Close Combat(but does not have priority, and lowers both defenses, so if you are slower or it does not OHKO, Infernape would probably faint)
- Flare Blitz/Gunk Shot
- Thunder Punch(A good move boosted by Iron Fist and if it Paralyzes, it lowers the oppoment's speed, making it almost outspeed anything)/Fake Out (A good move to deal a small amount of damage to hook a KO the next round)
I've hoped I've helped!