Actually I have realised that the Master cylinder is the same as the S14 (i think), but the pedal assembly is different. I don't suppose you have the pedal assembly to go with it? I have been sold an S14 one by the looks of things and it is slightly different. If I had both S15 parts then hopefully it will all fit well.
Ive never removed or tried to swap a S14 one so im not sure on the rod or fork length, but i do know that most jap cars have the same thread pitch on the rod and that the fork length is not the same for every car, so there is bound to be another car out there with a fork long enough, hunt around a wreckers yard, or just buy a new master cylinder from RHD, would be about 100 in your money.