extract from the wiki

I speculate that the US arm of DatsunSport quite possibly listed the stock F5W56A [wide ratio trans] as their Option 1 gearbox & the real Option 1, the close ratio F5C56A competition trans, as their Option 2 offering, thereby confusing the situation completely. [Author KB10]

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

Ok my fellow datsun tuners,i just baught an a15 engine.It comes out of the late model b310 that has the 56a Gearbox 4speed,Would the gear box be able to stand out for long?

My 1200 has the 310 model 56A gearbox 4 speed bolted on to my A12.I have already installed a 310 alloy centre diff which handles 125 lb.ft Which would be able to work on the A15.What are your options that i should use? Please help me out,ALL HELP WILL BE GREATLY APPRICIATED.

CHEERS DARREN..