What was nice when I did the 6" 3 jaw back plate the inset fit perfect the way it came. All I had to do was mark the holes and drill them. Using the DRO on the mill worked great for that. Now when I get the 5C chuck I will have to turn down the back plate for it is 6" and the chuck is 5". And the chuck mounts from the front I think. So I will also have to tap the back plate. So like you said its got to be nuts on. By the way LMS just put their 5c chuck on sale...We will be in business real soon I hope.
Matt
Matt