Yep NRBE is the normal GSR gearbox code. You'll have to remove the front gearbox mount from it as the GVR4 bolts onto the engine block anyway.
If your GVR4 runs a 3.545 rear diff ratio then yeah, the GSR gearbox and transfer case will work.
Alternatively, you can easily fit a GVR4 22 spline transfer shaft into the GSR gearbox so the spline count will match your existing GVR4 transfer case.
Strength between the GVR4 and GSR gearboxes seems much of a muchness.
They both have alloy shift forks for 1st/2nd and 3rd/4th gears.
The GSR initially was released with only a single synchro gearset, however, during sometime in the GSR run they started to get released with the double synchro 2nd gear.
The earlier GSR run also had shitty 'cardboard' type lined brass synchros which are crap, wear out quickly and burn up.
It is difficult to determine which GSR's ran the single synchro 2nd gear and also the crappy lined synchro's, but they are upgradable (bronze synchros and double synchro 2nd).
Recent comparisons Ive made between GVR4 and GSR/EVO internals are that the GVR4 seems to have thicker teeth on the centre diff and 3rd gear intermediate shaft (these parts are different to GSR/EVO and are not interchangeable). However, even though the teeth are thicker and the helix angle is cut differently on the GVR4, I am not able to determine if they are actually 'hardened' and 'stronger' than GSR/EVO intermediate shafts and centre diffs.
As far as transfer cases go the GSR uses a 1.09 ratio with a cast steel front case and a 23 spline shaft.
Ultimately I think if you cant find a GVR4 box then a GSR box and trasnfer case will be a good alternative.
GSR
1st – 12/35 = 2.917
2nd – 19/32 = 1.684
3rd – 26/29 = 1.115
4th – 30/25 = 0.833
GVR4
1st – 13/37 = 2.846
2nd – 19/32 = 1.684
3rd – 26/29 = 1.115
4th – 30/25 = 0.833
