You can re-cut the register on the chuck, either hold the chuck in a 4J or clamp some 1" rod in the chuck and hold that with your 4J. This should center the chuck relative to the collet. Should not be any significant deflection if you are not taking deep cuts. I was also considering something similar and planned to use the extra thick SB back plate. I am considering cutting the register deeper on the ER chuck and making some form of set-tru adjustable tapping for 4 small Allen set screws. When cutting cast iron in this setting, I have found that very sharp CCGT or WNGT inserts cut very nicely, I have used these on several back plates.