Edit: One of the biggest problems with Empire refills is they wipe out your current progress in food, iron, world energy, raid energy and alliance energy.
Wanted mission chests appear to give food and iron based on your total food and total iron storage. I am currently tracking some data about that.
In that case, especially if you have all level 18 iron storage, using gems to reset your Wanted: Monster mission chest and then hitting your highest XP, food, iron province may be your best bet for iron.