Hello and welcome to the forum,
I am sure more people will answer your question more fully, but for me the most obvious problem will be the relocation of the petrol tank. Have a look in your series 2, see how the petrol tank is configured in the donor car and see if you will want to relocate it into the series 3. Also, I would have a look at the rear suspension pick up points and probably a few other things. You will need to weld in some support brackets for the viscous coupling and if I recall on the series two there was a manual switch or lever into the gear box, so the gearbox will have to be changed. Also, on the P4 there is an ECU to sort the rear wheel brake decoupling on braking or something like that, but I am not sure if there is the same on the series 2 4x4, it was a simpler system if I remember.
It can be done I think, but the system is not as good as the P4.
In answer to your question have a really, really good look all round your series 2 4x4 and then a really good look around the series 3 - spot all of the mechanical, bodywork (welding in brackets/ boot floor for petrol tank / suspension pick up points) electrical connections etc - and then decide if you want to proceed.
Is the series 2 driveable? - that should help you decide whether you want to move the 4x4 across.
Remember the series 2 system was I think not design for permanent use - more off road as it was missing the centre differential (from memory) that the P4 had - hence the name. If you put the 4x4 system straight in without a / the P4 centre diff - you will get transmission wind up on bends and I would suggest that could be dangerous!!! The 4x4 was more agricultural and if you read the system descriptions it says I think to use the system only on loose ground - but not totally sure about that - it was a long time ago