I believe a GTR has the same floor plan as a gtst, so by rights the belt mounts will be the same, the only issue would be getting the correct seat (R32 GTST).
I could fit 3 kids in the back of my 33GTST up until the eldest was about 11, when the kids didnt fit anymore I ripped out the seat and put in a half cage, LOL.
Word off warnining though, unless its engineered they may not be covered by insurace.