You're right, back in the day, most of the GTR's though were either 2.6, or 2.8 only. RB30's werent THAT common.
I was in this exact boat when planning for a gearbox. I went with a 3.2 route. I sussed out all my options and talked to a tonne of people around what options there were for GTR's(excluding auto's here):
* Getrag 6 speed
Personally, I wanted peace of mind hence why I didnt go for a gearset or 6 speed. I mean, by the time you've done either of those 2 options you wouldn't get much change from 15K right? Plus, I dont know how long they would last with the fat torque of the 3.2...I also scoped out the sequentials out there, and in the end I went with the Albins as to me it seemed like the pick of the crop(Not saying the other Seq manufactures are shit, but its just my opinion). Plus, the Albins kit literally comes with everything minus the clutch.. no faffing around looking for tailshafts/transfer cases etc.