hi andy,yes you are right the chester model b chuck is bolted to the main spindle, any other chuck will fit providing that you have the appropriate back plate with a register of 72 mm for mounting on the spindle. i have had a model b myself good machine for the price,very adaptable.i built a 3inch traction engine with it. i then upgraded to a chester cub 630 lathe .
I made a new backplate to attach a 5" four-jaw I got from Chronos about five years ago. Works like a dream.
Method: got a disk blank cut locally, marked out the three mounting holes for attachment to the Chester, then the four holes for the chuck. Drilled all the holes. Then mounted the disc on the headstock with four longer screws and machined the recess and centre hole. Reversed the blank so it fitted in the correct way and machined the edge. Removed; counterbored the holes for the chuck. Attached to chuck and it was ready for use.
Since most 4-jaw work is at lower revs, the extra weight hasn't shown up any problems, even though I prefer the 4-jaw and use it more than the 3-jaw.