I have a SB-9 and was given the same advise about a six inch chuck. I bought it with two six inch chucks, three jaw and four jaw. I had no problems with those original six inch chucks and I did some math that showed me that there were no limitations due to the size of a six inch. I could fit any sized (diameter) work that a five jaw would hold and some that it would not.
Another argument that I heard against the six inch was the weight of the chuck. A SB-9 has a journal bearing which has 100 times more support area than a ball/roller bearing would have. The chuck weight may be a factor on a lathe with ball or roller bearings, but for a journal bearing it is a piece of cake.
I purchased a new, six inch, three jaw with reversible jaw tops. It has worked just fine. I love it.
As for machining the back plate I did this:
1. Mounted the blank in my four jaw and centered.
2. Took a shallow cut to make it round and a facing cut.
3. Reversed it in the four jaw gripping it on the freshly cut OD.
4. Turned the rear part of the back plate as needed (I had made a drawing but that is optional.)
5. Drilled and bored the hole for the threads.
6. Bored what is called the "register". This should be somewhat larger than the diameter of your spindle that it will sit above. It does not need to contact the spindle as the thread will do the centering.
7. Cut the internal thread. I had made a replica of my spindle threads to test it with, BUT
8. Without taking it out of the four jaw chuck I took the chuck with the new back plate on it off the spindle, reversed it and tested the thread I had just cut. I had to take off a bit more and test it at least twice more. This ensured a good fit on my spindle and kept my threading tool in sync with the already cut thread so I could just go right back to making the thread a bit deeper. This worked like a charm.
9. Only after I was satisfied with the fit on the spindle, I removed it from the four jaw and put it on the spindle.
10. Then I finished it to fit my new three jaw. This ensured almost perfect concentricity of the mounting thread with the surfaces that located the new three jaw.
As for the holes, I did not use a through-the-hole transfer punch. With tapped, blind holes in the new three jaw, that would have been difficult, no IMPOSSIBLE. You could use those short, threaded punches that fit in threaded holes, but I did not have any. What I did was to install the six mounting bolts in the holes in the chuck and take three outside and three inside dimensions. I added all six of them together and divided by six. That, hopefully gave me the average, center-to-center distance between opposite holes.
I then put the back plate on my rotary table on my mill and centered it. I had noted the center position of the RT before mounting the back plate so I returned to that position. Then I moved the X axis for half the center-to-center distance I had found above. I did drill the holes somewhat oversized so there was some slop to allow for errors. It worked just fine.
As an alternate if you do not have a RT, you could use a pointed lathe bit to mark the needed diameter on the back plate while it is still in the lathe (step 4-5 above). Then you could set a pair of dividers to the radius needed and step off six divisions on that circle. Go around once or twice to test your setting and adjust the dividers as needed before making the final marks.
It's your funeral but ;-
A 6'' 3 jaw is over size for a 9'' SB, ......6'' 4J lightweight yes, 3 J no, 5'' is more than large enough