Neither of them will fit, due to the rear suspension transverse link that holds the back of the 'A' arms together. There is no room for the finned area of the diff cover on a standard 240/260Z.
See this pic, item number 3. http://www.zhome.com/ZCMnL/tech/240ZRearTransversLink.jpg
Note: this is the early 240Z before they moved the diff further back, and closer to the transverse link to reduce the angle of the driveshafts.
To use a finned cover you would have to do something like this:
http://arizonazcar.com/diffmount.html
As dat2kman said, tapped standard cover and a cooler is the cheaper option.
Also, not too sure about the legalities of fitting and AZC rear end in Aus. Mr Plod may not be too impressed with all the bling. Talk to your engineer or relevant government authority before ordering, if planning to use this on the road.