give it a go. I'd probably cut off the section you need then slip it and the old yoke onto the shaft splines so you can put a couple of spot welds on to locate the new section properly.
i can't see anything other than a slight imbalance being an issue but if you weld it neatly and then linish it back so its all even it should be ok.
What I did on my shorty was I went down to the wreckers and got an old subaru touring wagon rear shaft which is same diameter only cost ten bucks, then machined off the splined shaft and uni joint welds ends as they are like a plug that plugs into the the hollow shaft and rewelded them onto the suby shaft which I had cut to the right length. quick and easy and not dodgy.
