[matto]

Remember that a lighter graphite shaft will alter swingweight and swingspeed. Also make sure that shaft length and thus lie angle are equivalent to the corresponding irons and the transition is pretty easy.

For now, I'd say (without any extra information other than an 18* 5 wood) that with the same length shaft in graphite, you could go with 22,25* and I'd say that those two would fill the gaps pretty well.