I dont think you'll have an issue Ricky, if the mounting points change that means that the chassis had to have changed (relocated mounting points for rear clusters) I cant see Lancia doing that just for a change in manufacturer of rear lights/lenses. Lancia were a bit bonkers at times but not that crazy. Its probably just the lenses themselves that are made by two different companies depending on early/late production.
I know that Siem is part of Magneti Marelli, who also own Weber, A-L, Carello, Solex, Veglia Borletti and others (all names that are synonymous with italian motors) - my guess is that Altissimo either got bought up (by MM?) or simply went to the wall because MM was so big and so "in bed" with Fiat, Alfa and Ferarri.
Anyone know the Altissimo history?