> I'm wondering if the size is different between the US Z model and the GX model

The answer is in the Which Carburetor article.

Two carbs don't have much more airflow than one due to the design of the intake manifold, so it doesn't matter much whether you use one or two. Just use whatever the manifold is designed for.

And no Datsuns came with SU carburetors. They are Hitachis.