I did a few of them while working in an auto parts store that also did machine shop work--they do suck to put together,there is a spring inside the stub that the ball rides on that makes putting the caps onto the U-joint crosses a 3 handed affair--most of them aren't worth rebuilding because that stub often gets galled and worn down and the ball will wear it out rapidly ,leading to early failure...
It needs to be turned down and spray welded and turned smooth to fix it right,by that time you'd have spent more than a rebuilt driveshaft with all new CV parts installed costs...most customers opted to buy a rebuilt when they found out the price wasn't that outrageous...