Round two
The reason for the sockets on the extension piece being a tight fit & so deep is to stop the shaft moving around during welding, i have them made tight enough that i usually need to heat the extension & cool the shafts to press them together, ive not had one move out of alignment .....yet
With a hardtail the shaft can be any length you desire as long as it runs parallel to the ground (12' gs 850 anyone ?)
on hardtails i usually run the shaft open & just use the back end of the arm where the diff mounts welded to the new framework
No machine shop or trick kit is needed to extend the arm itself just some carefull cutting, a tube bender & some decent welding

.....
For obvious reasons shaft drive arms are reasonably straight drive side, this makes it a simple matter to cut the thing in half weld in a joining slug to both halves then join them together with the correct dia tube, once linished back & coated most people wont see the joins
Another way is to cut the formed parts from both ends of the arm & replace the whole length of tube, the tricky part is keeping it all straight while you weld it up but a simple jig & some clamps holds it all fine as long as you are carefull
The other side can be a little more difficult as they often have a curve in them...
I there is a straight bit that wont either spread the arm (or narrow it) once the required stretch is put in then thats the place to cut & follow the steps perfected on the shaft side, alternitively you can remove the whole tube & make up another longer one to suit
hope it helps guys