Dear Malcolm
The way that they do it on most vehicles is either straight off the the end of the output shaft at the back of the gear box or a bevel gear off the differntial crown wheel, whichever is at the furthest point. With the Fiat engine being at the rear either would be easy to do and run a drive shaft forward to another differential on the front axle.
There were about four or five Italian compamies that offered 4x4 Fiat 500, 600 or 126 derivatives, so it could not be too difficult. They were mostly used up in the mountains by small farmers, who could not afford Land Rovers or Jeeps.
Chris Thomas