Update - This swap is much easier and better for other reasons if you use the Modern Motorsports "Z CV Halfshaft Adaptor Kit". When you purchase this kit, you send Modern Motorsports LTD. companion flanges that match your stub axles (either the 25 spline 240Z/early-260Z version, or the 27 spline late-260Z/280Z version).
The information below is still presented to show how the conversion was done on my Z. I recommend the Modern Motorsports CV Halfshaft Adapter Kit for any 240Z through 280Z with significant power increases over stock.
The parts to do this swap became available, so I upgraded to slightly larger 280Z stub axles. The 280Z stub axles are slightly larger in diameter at the spline, and have two more splines (27 vs 25), so the 280ZX Turbo companion flanges won't work. Instead of having a custom splined companion flange made, this conversion adapts the stock 240/260/280Z companion flange to the 280ZX Turbo CV-joint half shafts through a two-piece adapter.
One piece of the adapter is a flat round plate which bolts
to the stock 240Z, 260Z, or 280Z companion flange. (The 240Z and
early 260Z is more easily converted to CV-joint half shafts using the 280ZX
Turbo companion flange, as outlined here.)
The second piece of the adapter bolts to the first piece (using three bolts)
and to the 280ZX Turbo CV-joint half shaft using the stock 280ZX Turbo
bolts. The second piece of the adapter is basically a short "spool".
14 - Nissan 280Z companion flange-to-U-joint style halfshaft bolts,
14 - Nissan 280Z companion flange-to-U-joint style halfshaft nuts, PN 39727-N3000
8 - Nissan 280Z companion flange-to-U-joint style halfshaft lock washers, PN 08915-2401A
12 - Nissan 280ZX companion flange-to-CV-joint style halfshaft bolts, PN 39606-W1202
12 - Nissan 280ZX companion flange-to-CV-joint style halfshaft nuts, PN 01223-00041
(Note: All bolts, nuts, and washers above are NISSAN OE for either the U-joint or CV-joint connections at the stub axle. They are shear bolts, not just typical through bolts.)
6 - regular 10mm (or 5/16") flat washers
Here are a few pictures of the complete adapter bolted to the 280Z companion flange, as assembled on the strut housing:
Click picture to left for a larger, annotated version
Here are drawings of the two pieces. Note that there is a large amount of lathe work to hog out the volume between the ends of the "spool". The first drawing is the flat plate that bolts to the stock Z companion flange (after it is bolted and torqued onto the stub axle in the strut housing). The second drawing is the "spool".
After the 280ZX Turbo CV-Joint half shafts and one each of the adapters described above were obtained, and the late-260Z/280Z stub axle and companion flange were installed in the rear strut housing and torqued to spec (using the 280ZX lock nut, PN 43262-W1200), I bolted the 1/4" flat plate part of the adapter to the companion flange, with the bolt heads towards the inboard side. The bolts (4 per companion flange) are PN 38628-E4100, the nuts are PN 39627-N3000, and the lock washers are PN 08915-2401A. I used blue, medium strength Loctite on the threads and torqued to spec.
Next, I bolted the 2nd "spool" part of the adapter to the halfshafts, using the 280ZX companion flange-to-CV-joint style halfshaft bolts (PN 39606-W1202), 280ZX companion flange-to-CV-joint style halfshaft nuts (PN 01223-00041), 6 per halfshaft. The bolt heads had to be towards the inboard side, next to the CV-joint. Two (2) of the bolts (per adapter) needed to be slightly shortened on the threaded end to obtain clearance between the end of the CV-shaft bolt and the head of two of the companion flange-to-flat plat adapter bolts. See the drawing above for the locations/orientations of these bolts. Since the 280ZX Turbo CV-joint to compainon flange lock washers will not fit between the ends of the spool, with the nut included, I used blue, medium strength Loctite.
Then I slipped the halfshafts (the shorter shaft goes in the left side of the diff - it also has the shorter splined length) into the R200 differential. With the strut and lower control arm bolted into the car, I had to remove the top of the strut to swing it outward to get the two parts of the adapters together. After getting them engaged, spinning them to align the 3 holes, and reconnecting the top of the strut to the unibody, I installed 280Z companion flange-to-U-joint style halfshaft bolts (PN 39628-E4100) through the 3 holes, with the bolt heads toward the outboard (wheel) side. I used regular 5/16" flat washers and 280Z companion flange-to-U-joint style halfshaft nuts (PN 39727-N3000). Again, blue, medium strength Loctite was used.
I'll just say that even though I haven't driven the car with these parts, they were used in a V8 260Z (550 hp) for several thousand miles and several drag and road race runs - and they didn't look any worse for the wear.