The 1200 rear axle assembly is wider, so fitting one, sedan or ute, to a Datsun 1000 is NOT really a proposition.
Like PDS said, the 1200ute diff is larger than passenger models.
The 1200 passenger diff is not "the same" as the 1000 one, but it is "interchangeable" The 1000 ratio is 4.375 to 1 for utes & vans & 4.11 to 1 for other models. All of them used an alloy third member housing.
The 1200 passenger models used an iron housing with a 3.9 to 1 ratio. Using this is recomended when upgrading to a 1200 or larger engine.
Chris