Dimo, I don't understand your obsession with this Zed of yours...
I know it's very original, but IMHO your taking it too far.
If you can find NOS original seatbelts (highly doubtful) then thats ok, but an old set are downright dangerous...
Choke handles & cables are available brand new from places like AllZparts.
The Air-con you can use from any model 240/260Z as it is all the same - you could even use the basic system out of a early skyline. You'll never see any of it & NO-ONE will ever be able to tell.
Have you bothered to call any of these Zed retailers/wreckers places to find out whats available?