When I converted my 914-6 to the 108 mm CV joints from the ’69-’71 911, the length and size of the splines fit perfectly.
Keep in mind that even a 914-6 has the similar very small CV joints as a 914-4.
However, the 914-6 axle shaft splines are compatible with 911, the 914-4 splines are not.
(Same issue with front torsion bars.
)
Best,
Grady