Now I see your need for a larger welder...CV Extending...um, well, ahh... rather then give you the hundred reasons you shouldn't be trying this yourself, I ask "what's the plan?"
I have a buddy who worked rebuilding CV's for a large remanufacurer for a number of years. I'll bug him a bit about this, I'm sure he will have some good input, and he's local and he has an Electric Jeep project in the works
My first thoughts are that you'd have to have the OD of the CV's turned down a bit to make a consistant diameter, then have a sleeve made so they can be pressed in to assure concentricity and cylindricity. I have no idea how thick and how long that sleeve should be... There are specific techniques for welding this type of joint as well. I'll look in my handbook for some inspiration. Yes, I'd want a bigger welder for this. It'd cost about 6K for one big enough...