Re: how to fit speedo transducer?
It sounds from your description you are after the plastic spindle that goes into the gearbox - so as the gearbox turns it turns the spindle which generates the electric pulse in the sender. Did one on a Tipo some time ago, but they are the same as the Alfa.
Take off the large plastic part (the sender) (that's the bit the multi plug clips into) that should just screw off. Then the allan nut you mention should release the collar that the sender screw onto, that also holds the plastic spindle in place and is about an inch and a half long in total. Your question asks if this will drop into the box - the answer is no. Once the collar is removed the spindle will lift out. The bottom end of that sits on a plate in the 'box so that will not drop either. As in the usual Haynes speak, the refitting is the reverse of the removal.
Hope that helps....