As it is a steel steerer tube it is easier to work with. With an insert, sleeve and a decent weld, it should be a reliable extension.
The steerer tube at the moment at it shortest point (un-even cut) is 125mm. I can get you some pictures if are still interested.
I will look for the headset tonight for you.
EDIT: I have just measured the headtube on my frame and it is also 120mm. So as I said earlier, I would recommend a 190mm+ steerer tube so that you aren't restricted as to what headset and stem you can use.