The chance of flinching when using Iron Head would still be 60% with the flinching item. The flinching items only add the chance to moves that don't already have a chance to flinch, and since Iron Head already has a flinch chance, the item would do nothing.
If you use a different move that can't already flinch, Serene Grace increases the chance of King's Rock/ Razor Fang flinching to 20%. Up to you whether it's worth it or not. Most paraflinch sets just run Leftovers.
See the King's Rock box here for info on the items.