I Ok, I know a lot of the silvia guys do the RB25det gearbox conversion, but there are a few potential differences. First up, how much power are you planning on running and what size tyre are you going to use?
I mean a lot of silvia guys complain that the SR/RB20 gearboxes aren't strong enough, but then you have to remember they typically run 250+rwkw (about twice s13 factory power) onto 295s and then put in heavy brutal clutches as well.
If you are running less than those figures I would strongly recommend sticking with an sr/rb combo box.
Here is a photo of both (RB20 box is on the right)
So the body of the rb25 gearbox is significantly larger too, but the shifter looks to be a little further forward.
Keep in mind that I had to modify the tunnel/firewall to fit the sr/rb box in recently. If you stick with that box you don't NEED to do a full boxed up tunnel, but I can't see any way that you could avoid it with a rb25 box! Also from the research I have done the RB25det is about 20-25kg heavier
Finnally if you do choose to go the RB25 box make sure you get the rb25DET box, as apparently some rb25DE cars came with the same box as an rb20det.
Hope some of that is useful