Having 3 steering gear in a row all give the identical problem sounds very much like something besides the steering gear. But then I don't know what car parts places really do when they receive a "bad" unit back. Probably 50% chance it goes back on the shelf for another person to buy, especially if it looks OK from the outside. I guess I'm cynical.
I don't know what kind of slop is added when a steering center link is on backwards, but people do put them together that way accidentally and then complain about terrible steering, and the person experiencing problems posts pictures of the steering setup including the center link and people reply "yep that's the problem". So pictures are good. Just an idea.
If it were mine I would pull the steering gear off the truck and put it through the paces of adjusting the preload using the procedure used when seals are replaced. This can be done by steering the gear to off center, tightening down the adjuster plug, then backing it off by 1/2 inch travel of the threads or spanner hole. Then after steering the input shaft through the entire travel if you feel the over-center preload is non-existent or excessive, adjust over-center preload. If you can feel slight over-center additional resistance it is probably good.
Or you can adjust the preloads by the book, which is confusing. Here is information in a thread I started when exploring what to do and studying the situation. The book method is in the thread, as well as the shortcut method. Mine is 2 wheel drive, I don't know how much of a difference that really makes. I should say I had minor slop to deal with, but nowhere even close to a turn and a half of input shaft slop.
http://67-72chevytrucks.com/vboard/s...d.php?t=812162