Use the 26 loom, youll have to if you running the twin turbos anyway, so youll need the resistor pack wired in for the injectors.
You'll need to modify the gearbox cross member, or make a whole new one for the RB25 box to fit, and then get the tailshaft made up.
All these little things are covered in various thread around the place (RB25 box in 32 gtst, RB26 into gtst, RB26 maintenaince, etc) search up.