Had to beef up a truck and we decided to put extra axle on. Think first decision you have to make is whether you want either -
1) 6 wd (a much bigger job)
2) 4 wd and a dead axle (still a big job)
Usage would dictate - if its for load bearing only then 4wd would do the job, wouldn't imagine you would want to be off roading in this as it would bottom out all the place!
But if you wanted to go for 6wd, and have serious usage on it - then just run it off the rear diff. But you'd have to beef that up, the rear prop, the vcu, the transfer and the gearbox. And have to look into getting it properly set up for wind and vibes.
The actual welding the extra length on to extend the body i couldn't give any opinion on (my welding leaves a lot to be desired!)
What are you wanting to use it for?