DO NOT PUT A 2.5 BOX IN IT!!!!
I'm running a 2.5 box with a 3.2 engine and it is USELESS in the wet. Even in the dry I nearly always use 2nd to pull away and its caused the clutch to wear out in 20k miles. 1st Gear is simply too short for the GTA engine.
I'm currently investigating a solution in the form of a 3.0 166 gearbox - Its an oldish thread but I am still working away on it. (Both gearboxes are sat in the garage doing nothing...)
Alfa Romeo forum: 3.0 166 gearbox in a 156?