Making aftermarket rails is not that hard but very time consuming. I have fitted aftermarket seats into my 1000. The biggest issue was finding a seat of appropriate width so it does not interfere with the door jamb and b pillar area.
Most seats will have a adjustable rail that is welded or bolted to the base of the seat, this needs to be removed and likely can be thrown away as it won't be even close to datsun size.
Measure the distances between seat bolt holes in the car and weld together some flat bar steel into a frame to bolt into the car as your new custom rail. Then the seat can be bolted to this custom rail.
I used this r31 guide as a bit of help the first time I had done it. I have done it in two different nissan/datsuns now with good success:
https://forum.r31skylineclub.com/index.php?topic=108649.0