Hi Mate,
Ive done this conversion to my car which is a R33 GTST Sedan. I changed Engine Wiring loom, r32/33gtr ecu or u can go PFC, I went with the R34 GTT gearbox with an r33 gtst manual tailshaft, but you can use the r33 gtst gearbox.
I think the only difference in between the 2 gearboxes are the R33 Gearbox is a push type and R34 gearbox is a push type.
You will need to modify the front sump, take off and weld the holes where the front drive shafts come out or you might be able to find one that has already been welded up.
You will also need the power steering pump, alternator (might be able to use the same ) I think the reason you need to change them is because the width of the pulleys and the fan belts are smaller on the rb20/25.
You will also need the return bov piping unless ur planning to run the pfc djetro or a mafless tune.
Thats all i can think of for the moment