R180 to R200
The O.E.M. 240Z differential is an R180 (180mm ring gear) type. This differential is smaller, and weaker than the latter 280Z differential, which is the R200 (200mm ring gear) type. They can be differentiated by the different rear covers (the R180 has a square looking rear cover using 14mm bolt heads, and the R200 has a more round look using 12 mm bolt heads), and by the way the axle stubs are attached. On the R200, the stubs can be removed by simply prying them out of the differential using a couple of screwdrivers. The R180 though, has a long bolt going through the stubs, holding them into the differential, and also, there are removable bearing carriers on both sides of the R180 differential, that do not exist on the R200 differential.
The swap from the R180 to the R200 is very straight forward, being the two differentials have exactly the same mounting arrangement. The R180 axle stubs bolted to the half shafts must be replaced with the R200 axle stubs. Everything else will remain the same.
Note: If you have a very early 240Z, I would highly advise you to use the latter 240Z (and later Z's) crossbar supporting the rear of the differential and the later front differential mount. In doing this, the differential will be moved rearward just over an inch, and thus aligning the differential stub axles to the wheel axles. The very earliest Z's had the differential forward of the ideal location, causing some vibration due to the lack of good alignment of the half shafts (U-joints don't take misalignment as well as CV joints do). Datsun remedied the situation the next year by moving the differential back to a more ideal location in which the half shafts exited the differential perpendicular to the centerline of the car, instead of sweeping rearward.
Note: Even though the exterior of the Z and 280ZX differentials appear to differ, the 280ZX differentials (I've been told by a knowledgable source) have the same internals as the R200 Z differentials, with the exception of the Turbo model, which uses larger ring gear bolts then the non-turbo models.