Subaru WRX STI GDB model seats are a good choice. They are not as wide as the original Datsun buckets and can be installed without cutting out the Datsun front bulkhead/web where OEM seats mount. Retaining the bulkhead/web and utilising the other OEM seat mounting holes and seat belt mounting points, albeit with some modification, makes it easier when it comes to obtaining engineering certification. I wanted seats that gave greater front-to-rear adjustment than the originals, they had to be firm and comfortable, driver side height had to be adjustable, and had good bolsters for lateral support. Installing these turned out to be something of a chore, but the final result was well worth the effort. If you choose to do something similar, this posting may save you time and headaches.
The seats provide same seating height as originals, though you are unlikely to be able to remember what the original seat height was when your Datto 1600 was new, because the old springs in the Datto seats would have have long since sagged. Seating position of the installed STI seats seems higher, but it is not.
The approach involved trimming down three of the four corner mounting brackets, but not the inboard rear. The inboard rear one was retained because this is particularly strong and Subaru deliberately integrated the seat belt mounting into the seat. In my case, I am using the original Datsun floor attachment point for the inboard rear seat mounting point as well as to attach the Subaru seat. After trimming the Subaru mounting brackets (and enlarging a hole in the inboard rear one), 16mm CrMo tubes were welded across both front and rear to ensure seat rails would be fixed together, and precisely located relative to each other. Note that without these tubes it is hard to get exactly the right spacing between rails, and it makes the whole installation stiffer and stronger.
One bracket for the outboard attachment of the seat to the car had to be fabricated and another for the inboard rear attachment. From an engineering viewpoint it is important to retain the Subaru OEM brackets with as little modification as possible, particularly as they are riveted onto the rails. It is practical to weld to the rivetted-on bracket without weakening the rails. Note that riveting is used here because it is stronger and more reliable than welding and allows for steels that are easy to weld to be attached to others that may not be so easily welded or may need pre- or post-weld heating or some other special treatment. In the case of three of the four Subaru OEM mounting brackets, where these were cut down, a steel tab with a bolt through it was welded on to facilitate the seat being attached to the vehicle. All the welding described involved steel parts that could be readily welded without causing problems. Photos tell most of the story.
Details of passenger side are shown. The driver's side is similar, though the height adjusting mechanism makes things a little more complicated. In the case of the passenger side it is very easy to unbolt the rails from the seat and do the fabrication and welding, and to the test fitting without trying to juggle the whole complete seat. However, this was not so easy for the driver's side, so the passenger's side became the test case and when completed it was a matter of replicating the modification process for the driver's side. All welds were done carefully with a MIG, but because the driver's side the rails remained bolted to the seat, and a wet towel was carefully located to protect the fabric and foam padding. This ensured that my seats didn't end up looking like Chinese New Year fireworks.