Once you assemble the upper bearing and balls onto the worm gear and then into the tube, you can then install the spring, sleeve and woodruff key and that should hold the balls nicely in place.
I was just about to say that this is what I used to do, and I got to your last post. Works a treat.
A couple of other notes:
I used to remove the O-ring and use a conventional oil seal. (you have to pry out the steel washer and bore the recess (or find a seal that fits) to suit an appropriately sized seal.
You must check for wear/play on the main sector shaft (both at the lower and top bearing surfaces). If any significant wear/play is detectable, you must rebush these inner bushes after regrinding the shaft journals (a straightforward job for any competent machine shop). If there is any play here, you are wasting you hard work elsewhere. The clearances here should be around 0.0075". Best acheived by honing!
The adjustment of shims for the steering column shaft should be done by feel of hand before assembly with the sector shaft (a dummy run).
The final adjustment of shims on the top oval cover (before installing the heavy spring) I used to do by installing the pitman arm and then mounting it in a vice (on the pitman arm), installing the steering wheel loosely, and then while the steering wheel is in the STRAIGHT AHEAD position, checking for play. You should aim to adjust the oval plate shims so that you get absolutely minimal play in the wheel but no binding. Then install the heavy spring. This procedure ensures that the box will have no play, but remain as frictionless as they are reknowned to be.
I have done a bucket of these boxes that people have otherwise said are "unrepairable" etc...
I have also replaced the cups and ball races with tapered roller bearings in one case when I could not get a good replacement main steering shaft.