There's a difference in the top diameter with the NG900 struts as I recall. It's more than just a minor length difference (which I think is also there) .Thanks Jeremy. Not US.
I should have bought a new one when our dollar was higher.
What I do know is that Bilstein has different part numbers for both but also that they'll do that if there's 3 mm length difference.
The inserts. I don't know if anyone has ever measured the assembled struts... must be very close. You can do a conversion of the springs, but I don't think it will save you any money to do it: http://photo.platonoff.com/Auto/20070511.Saab_NG900_Eibach_Springs_From_a_9-3/Struts or inserts?