Well thanks to Scott at Venomcobras I have finally started this delayed project. I have an R33 DE 3.58 ratio diff to replace the existing R33 (4.11).
This should all go in with relative ease, although I will replace the rubber bushes on the nose of the diff with solid ally ones. I believe that the relative size of the old and new diffs are the same (both R33)
The predicted hassle is the half shafts (well described by Paul and Warren above). They have a different bolt pattern (6) from the existing ones (5). In addition they can have a different number of splines on the shafts themselves.
I can get some different shafts I guess but some have used an adapter of some kind. I am wondering how that works? Surely it would have the effect of lengthening the half shaft?
Replacing the half shafts would seem to be the best option - I would, of course, then need to match the flanges (above) as well as getting the length and spline pattern right.
Any opinions welcomed!