Ah, in that case the answer is yes and no. It will phisicaly fit on, but the RB25 crankshaft has the small oil pump drive a-la early GTR engines. This is fine on standard pumps but an uprated one will shift quite a bit more oil, and put more load on the driving surfaces, the flats of the drive on the crank nose only have around 3mm of contact with the oil pump. If you're going down this route you really need to fit a oil pump drive collar to the nose of the crank..
and fitting the oil pump is an engine-out job. It *has* been done with the engine in, but the hassle is such that it's really not worth it, and you surely won't save any time. Imagine - you've dropped the oilpan an inch or so, then you're both trying to make sure you've cleaned out the old silicone and applying enough new silicone...while under the car on your back.