Many Australian Datto owers use Diffs from box-like commercial vans. Also use diffs from bigger cars like the Holden Commodore which uses a Borg Warner BW78 type diff. The 4 door R31 Skyline also has a BW78 diff, so the parts are very common here in Australia.

Unfortunately, there are not many choices of big diffs from other cars that are narrow to suit a small Datsun without them needing shortening.

To shorten a diff, you really need to make up a fabricating jig, to hold the parts together while you cut and reweld them.

For shortening the axles, this normally means getting two long side axles from your choice of diff, and shortening both, to provide enough diameter on the axle to cut a new spline. I cut new splines on axles using a milling machine, but it uses about $50-100 in cutting tools alone to complete the job, as cutting the splines on the hardened steel axle is bad for the tungsten carbide cutters.

If you are looking for a cheap alternative, finding a Z18et and using a Datsun 610 (180B) crossmember in the 120Y would make it a direct fit.

Given your remote location (from japan) and limited tools/equipment, you might be better to find a L20B and get dual webers or consider some form of turbo system.

The translation software has more spelling mistakes, but it's quite easy to understand.