You dont want a long spline for the rear.
You would be plenty happy with a cv, the standard slip and thicker tubing.
Get your cases installed and then measure the length and angle. Then call a shop and order a rear shaft. You may need to roll your pinion up towards the tcase depending on what your measurements are but the shop would be able to tell you.
Now I'm off to go build some.
![Gap :gap:](https://board.marlincrawler.com/Smileys/marlin/gap.gif)