Pfitzner Gears in Sth Australian list a conversion for Nissan 5-speed and the R35 6-speed transaxle but have no announced plan for the 6-speed trans in the Z34.
You can expect this to cost well north of $9K (AUD) plus labour to strip and re-fit .... based on the PPG Gearbox web-site prices for the 5-speed.
The other alternative is to get Holinger to put a box together - gear ratio choice found here:
http://www.holinger.de/tl_files/prod...6_ratios_E.pdf
but expect that to cost north of $20K (AUD).
Remember that you need to change the layshaft gear as well as the main-shaft gear as they need to be compatible
Whatever you do, it will not be cheap.