The shafts fit the differential so spline count is whatever it is. You can't choose. You might be confusing hub shafts with differential shafts.
Hub axles - http://www.modern-mo...stub-axles.html See the threaded end for the retaining nut?
A long-nose open diff is the stock diff for a 1977. Probably original. The diff that you're buying should be designed for Nissan's spines so the stock 300ZX CV joints, with splned axle attached, should work in the diff. Then you want the shortened axles to put those ends on - http://www.modern-mo...alf-shafts.html
You can also take the stub axles from your open diff and use them in the helical, with the u-joint half-shafts. You might do the job piece-meal to be sure the diff is right before you spend on the CV conversion. The u-joint halfshafts should work perfectly until you get the CV shaft parts.
You will need either the 27 OR 39 spline hub axles to use the CV shafts though. Because the bolt pattern of the flange needs to fit the CV flange. That's why it would be simpler to just install the helical and use your current u-joint halfshafts first. Then focus on the CV shafts with adapters for the wheel side.
The only aftermarket stub axles created for diffs are the Subaru axles to allow the u-joint halfshafts with the Subaru diff. The factory diff axles have been strong enough. http://www.wolfcreek...id=22&Itemid=31
I probably just added more RPM to your spin, but good luck.