Anybody know if a 280z rear diff will work on a 240z?

Same stuff.

The 75-78 manual trans equipped 280Z came wiht an R200, whereas the automatic versions came wiht the R180 found int he 240Z. Yes you can use the R200 but you'll have to change the "mustache bar" as well as the driveshaft for the 280Z which might need customizing, memory is a bit fuzzy on this one but there are a couple of input flanges for the R200 and I don't think either works with the 240's R180 shaft (if memory serves). Any decent driveshaft shop can make what you need or you take the trans, shaft and all form the 280 (also the clutch and flywheel you don't need them but larger area and holds more power)

Thanks for your input. Finally got the swap done. Just one major screw up, I didn't flip the mustache bar around and had to pull it all apart to turn it around.
In my case, the 240z driveshaft did fit ok.