Has any car maker factory fitted LPG? not as far as I know but I await correction. LPG problems are well known and makers don't want to get piles of warranty claims for an unnecessary fitment Especially Alfa.
If I was after a 156 I'd go for the deisel, they're very good..an AA man told me that other than the fuel feed pipe bracket occasionally breaking they're pretty much 100% reliable, sort that properly and you can weld up the bonnet and forget about it.