The adapters in your picture are for Porsche 930 design CV ends, not 280ZX or 300ZX. You can tell by the bolt pattern. Six evenly spaced holes is 930, three sets of two is 280ZX, and four is 300ZX turbo. The axle length depends on how much travel there is for the CV joints on the shaft ends.
No vendors seem to make just an adapter for an existing Nissan axle, although people (cygnusx1) have made their own for the 280ZX CV axles. The aftermarket guys all make hub axle and companion flange sets, which is somewhat overkill. There are plans for the adapters that will let you use stock 280ZX axles, on the Hybridz site.
http://forums.hybridz.org/files/file/3-cv-adapter-print/