I would take the back off the chuck and look inside to see what diameter the scroll centering boss is.
Alternatively, you could remove one of the pinion gears and check its overall length. Using your deductive powers, you can estimate how far the small end of the pinion is from the center of the chuck. Then you'd need to decide if boring the chuck is going to expose the ends of the pinions, or indeed, cut the ends of the pinions off. The former will allow chip dust to possibly enter the scroll gear; the latter will probably take the edge off your boring bar if the pinions are hardened.