I read what your saying but I am a bit confused.
Do I just stay with the 9" drum lower ball joints cause the later model ones are larger and have different spacing?
No, you need the 73+ disk ball joints because those are the only ones that will match the 73+ disk spindles you will be installing. The stud on the ball joint should fit your LCA because the LCA's didn't vary between 66 & 73+ that I know of, other than sway bar brackets were in a different place. Even if you have a sway bar, that wouldn't matter since you would keep it and your LCA.
Finally, the caveat. I did read that the only difference in the ball joints was that 73+ had thicker bolts between ball joint and spindle. That is why I bought 73+, thinking I could just add spacer collars for my 65 bolts (cheap bastard). However, I found the bolts were also farther apart. At least that was my measurement, but perhaps someone else has other info or experience. I have read people say that you could drill the holes in your 66 ball joint bigger, but don't know if they actually did so. Anyway, 73+ ball joints are the cheapest ones and most people would take the opportunity to install new ones.