Dale,
Don't give up on the back plate, you may well be able to recess your new chuck to fit! On a chuck, the area around the base where the spindle is rarely critical. Back plates are ideally cast Iron to prevent locking on the nose thread. If you make a steel spacer for a new chuck, a thin plate would suffice, pinned to the back plate.
I'm tempted by by your speakers!
Regards, Matthew