On the 4 cylinders all gearboxes are the same except for the final drives. Stay with original set up & fit the Alfa Q2 diff, available from Alfa as well as many Alfa parts specialists. Parts & labor to fit is around e650.00.
THen your transmission will cope with the mods you have in mind although I have not heard of the 4 cylinder FWD transmissions giving problems except for the Selespeed units.
The GTA147 breaks diffs etc but can be cured by the Q2, it's the torque that causes the failure not HP.
Hope this helps?
Ciao,
John
