If you had a 6v and 12v starter laying side by side, the 9 tooth gear on the 6v would be a slightly larger diameter. The other way of distinguishing between the two is the diameter of the armature shaft at the exposed bushing end.....the 12 v starter will be .470" while the 6 v starter has a shaft diameter of closer to 5/8".
You just need to make sure you match the 12v starter with the 168 tooth flywheel and the 6v with the 139 tooth wheel. Both flywheels will fit on any year of 235 and it sounds to me like you've got the smaller 12v starter bendix drive gear jamming on a 139 tooth 6v flywheel.
Should this be the case, you need to run a 6v starter on 12 volts which is a common practice. If you set it up with a foot-stomp starter, you don't need to make any modifications to the 6v starter. However, if you run a solenoid controlled 6v starter, you'll need to switch the solenoid to a 12v. I found that our local NAPA carries starters and solenoids for Chevy 6's.
Dave
