I would get the original diff rebuild and see how it goes. Don't underestimate it. They can take a little bit of punishment. And by the sound of it, you are going to keep the A12 which won't give as much torque as the A15 would.
The original A12 diff also has a very nice ratio.

Give us some more info on the setup. What are you guys going to do to the engine?
If she is going to use it for drag racing, what class is she going to compete in with the A12 sized engine?

By the way. I also have a stock standard A12GX. 1977 model and live in Bloemfontein.
Drop me a PM if you wanna chat private.
Regards