No fail, I've just been gone for the holidays!
I found out that the S15 Silvia SPEC R Helical LSD differential will indeed fit in the Q45 case.
Unfortunately even though the Q45 stub axles are also 30mm and 30 spline, like the S15, you will have to use S15 stub axle shafts because of the difference in length, or modify the Q45 ones to fit the S15 LSD unit.
Here is a useful site I found.