Merv, the backing plate comes off your R33 diff, since it is matched to the mounts on your R33 cradle. Put that onto the S15 centre if it does not mount straight up.
No need to buy anything other than the diff and axles as I understand it. The whole family of S13/14/15 plus R32/33/34 diffs use these R200 centres and are interchangeable - sometimes the diff swaps over by itself, other times like ours you need the axles from the donor diff. They vary by no. of splines, shaft length, flange bolts, etc. Checkout the Skylines Australia forum, it's a wealth of info.
I've been reading up on diff oils and you need LSD
oil in the helical (6spd manual) version. I'm getting Castrol EPX 80w90. You need 1.5L to 2L.