there's not much information on the newer Alfa's on this forum... I think you're better off, depending on where you are located, on English www.alfaowner.com or continental europe sites like www.alfaclub.nl
Peter is right, Alfaowner is the place to look and ask.
But, a 156 2,5 V6 Automatic is not the Selespeed box. It's a more conventional autobox, the modern type with electronic management and a facility to change gears up and down manually with a stick. It's the same box as used in the bigger 166, I think Alfa calls it Q-matic