The gearbox should hold out fine depending on how you drive it and how its maintained. The 56A is a more then decent but id opt for a 56a 5 speed for the stock gearing this transmission works very well for street, slaloms, high-speed time trials or rallies

But if your wanting something else its hard to find something great for racing for example in the tech wiki -
The F5C56A [JDM Option 1] is as rare as hen's teeth. These were racing parts, and thus are difficult to find and expensive. Some Japanese dealers fitted them to retail GX models for special customers. The Japan domestic model Datsun 1200 GX-5 came with the wide ratio F5W56A as standard. This was also available from Datsun Competition.

So it depends on what you want to do, and how you do it.

Im running a 5 speed 56a and it still doesnt miss a beat