The factory seat rails are not really detachable from the seats. For aftermarket seats there is an aftermarket base like the one in the link below that fits R32, R33, R34 C34 Stagea and probably a few other Nissans. Then you need a set of universal sliders.
http://www.trademe.co.nz/motors/car-parts-accessories/interior/seats/auction-501365460.htm