Ijust grabed my 82 print of the s30 body manual, looking at the PD section of the manual it states that manual cars have 3.7 R200 and autos have 3.545 R200 diffs. I have attached the schematic of the R200 take note of the extra size, strengthening ribs on the fwd section, and the large rear raised attach pad for the rear mount bracket R180 does not have this feature, also from the rear the R200 housing normally appears oval when compared to the R180.
Steve