Since you started a new thread, I will move my comment from the old one to here:
Repairable or adjustable? The early 7 tooth boxes (28-29) basically have no way to adjust the box itself, while the later 2 tooth box has several adjustments on the box. If your 7 tooth is worn, it almost has to be rebuilt because it is hard to compensate in the links for the lack of box adjustment. Many swap their 7 for 2 tooths because they are just a better design and fine for a driver. Or people swap with a F100 box, which is also great. Needless to say, unless somebody really botched the box (backyard mechanic fixes) it is rebuildable.
Now the parts not being available thing is a different story, one that I have never heard of before (but I haven't been doing this too long). Ford did have variations during the years in the boxes in design or manufacturer. I am too lazy to grab the JS right now so I will let someone else weigh in
