Bruno, have you done the swap yet?
You may want to get the eingine in place, then contact Inland Empire Driveline Service in Pomona California. They can make you either a chrome-moly shaft, or aluminum one. The Aluminum one is around $350, and they ship anywhere. All you would have to do is have the appropriate measurements.
Also, you may want to get in contact with Frank Poll, he is in Utrecht, the Netherlands, and his friend Xander, they are both Z-Heads and can help you with parts sourcing there on the continent.
The big "problem" with the swap is the ZXT coupe driveshaft is too short, and the input flange is larger than what is used on the 260.
If you swap the differential along with your engine and turbo tranny with it's larger output shaft, all you would need to have done is either have your existing driveshaft legnthened---or if your donor is a 2+2, shortened to fit the 260 Coupe. The cost for this is around $90 at IEDLS if you can't find a local engineering works to do this modification for you. The only parts you would need for the differential conversion are readily available here, and could be shipped easily, and cheaper than the driveshaft---contact me via e-mail off-forum if you need further details on going that route. Using the larger Universal Joints of the ZXT Driveshaft is a good thing for longevity.
You can post a query for Xander and Frank Poll (Frank280ZX) here in "Car Talk" and take the contact from there.
The link for Inland Driveline is: http://www.iedls.com
Good luck with your parts search!