Finished the diff off today.
I installed the S14 VLSD into the S15 casing so that the speedo will still work.
After it was installed, I got welding it:
There are 4 big holes to completely fill with weld so all the gears get welded together. It took an entire 5kg reel of 0.8mm wire, but didn't even finish off a small bottle of gas.
I then cleaned the diff out and fitted the backplate on and filled it with some Castrol 80w-90 GL5 diff oil. This will be changed after about 100miles or so.
The diff lifted into the subframe surprisingly easily using a trolley jack. Everything was bolted up and the car drives.
My first impression of a welded diff - it is ****ING loud, I mean really really loud! There is a constant whine and its pretty annoying, but I suspect this is because the previous owner must have had the diff shimmed quite a bit - I couldn't turn the output shafts in opposite directions even before the diff was welded.
But this is all pretty normal really. Need to go to pod and properly test it out!