My assumptions & conclusion were same as Davids.
You want 6-1 instead of 5-1, so making either the shaft wheel 20% bigger OR the take up 20% smaller should do. That will require re-spacing the shafts. I'd suggest a bigger shaft wheel (.9 inch) because it moves the shaft less, and also gives a stronger mechanism (smaller wheels = more force on wheel surface to transmit same torque).