OK - at the risk of sounding like a complete fool, I'll ask the question:
As I understand it, the 4006 frame and the 5906 frame are the same??
If that is true, why can't you use a 5906 slide and barrel with the 4006 frame? Or can you.

They're not really identical, the magwells are different. The nine mag is wider and the forty has a dimple on the front so they won't go into a nine.

Thanks Handgunner - didn't think it would work out, but never could find any threads or information on it, so thought I would ask a dumb question.
Born Ft. Dodge - been gone from Iowa for 40 years now, nice meeting you and thanks for the information.