The factory rails on either Stagea or GTR are not removable, they are part of the seat base.
You use the complete GTR seat and bolt it straight in, but they sit alot lower as noted above
All R32, R33 and R34 GTR are interchangable. R34 GTT RWD are not as they have a different floor pan.
There maybe a remote chance a RWD Stagea floor pan could be different like the r34 GTT RWD is. This would have to be confirmed. Also most people are not aware the RWD Stagea has different front strut towers than a 4WD Stagea, this is due to the different front suspension setup.