A rotary engine in my opinion would be pretty hard on a standard Datsun gearbox especially if you are using a turbo rotary. Wouldnt expect the gearbox to last long.

I would use the gearbox that would be normally attached to the rotary. Say its a 13b from a rx7 I would use the rx7 gearbox.

The diff in a 1200 is a little undersized so I would probally look at replacing it also. Doing it now would be the best option as you will need a new tail shaft.

The 1200 tail shaft is pretty light duty and will be the wrong length for the new engine combo. So upgrade it with a heavier item with bigger universal joints. (These are the flexible joints in the tail shaft which compensate for the angle and movement of the diff assembley)

A rotary into a mazda piston engined car, datsun, toyota, mitsubishi galant etc has been done before so maybe read up on old fast 4s magazine or simular for tips about what to use.

Have fun