no no no short axle is used, but never cut down and resplined......simply swap it to the other side and it becomes the new long axle
you can cut 10mm of spline off it to make it shorter, as there is 40mm of spline and only 25mm of it is used

then you cut the long axle down till you get the diff width that you want

only problem i can see you having is that the whole centre will be offset a bit to far if you use an un cut short axle as the long one, as with my diff the whole centre is now offset to the passengers side and the pinion is just off centre towards the passengers side
my diff is 20mm wider than standard 1200 size, so if you took the whole 20mm off the axle you were having resplined that would just push the centre over further
so then hte pinion would be a bit off centre making the tailshaft not straight
but i guess thats how it is in the skyline so it should be ok