Nadellas in the '65-'66 Porsche® 912
Nadella halfshafts were only used on '65 and '66 911s and 912s (starting in 1967 the halfshafts were designed for constant velocity joints). The output flanges coupling the transaxle to the Nadellas are a unique design, with a keyway and different bolting pattern.
Some potential advantages of Nadellas are:
1) They can withstand high amounts of torque. They were also used on the 904 and 906 racecars.
2) The joints are sealed.
3) If you enter a concours, perhaps you can score extra points for originality.
Some potential disadvantages of Nadellas are:
1) They are not constant velocity.
2) The lower joint is exposed to the weather, has no dust boot, and once it starts to wear out the Nadella must be rebuilt at a machine shop utilizing a universal joint with grease fitting.
3) The integral mechanism to allow axial movement is not symmetric, so when extended they may not be in balance, which could create extra vibrations.
To install a later transaxle with output flanges for CV joints into a 912 with Nadellas, or to convert from Nadellas to CVs, you'll need to assembly all the bits and pieces. CVs, half-shafts, and splined shafts to the wheels.
New Nadellas have not been available for many years, and Nadellas with the joints in good condition are valuable, so if you decide to upgrade, don't discard the Nadellas, consider selling them to another '65-'66 owner, or a 904 or 906 racer!