The ring gear size is not actually the same for 6 and 12 volt units. The ring gear inner diameters are the same for foot stomp and solenoid starters of the same voltage, though.
When I installed the 261 (uses the same flywheel as a 235) in my truck I wanted to use the 216 bellhousing and footstomp 6V starter. Since my old flywheel had a 9-1/4" clutch I wanted to use the 261 flywheel, but the ring gear inside diameters were differant. The 216 ring gear had a smaller ID, so I had a machinist turn down the 261 flywheel to fit. It worked just fine. The 12V ring gear will not fit a 6V flywheel.
You will need to count the teeth on your existing ring gear and order/obtain one of the same count. The 12v ring gear is of finer pitch. I have a spare 12V ring gear, and they are pretty easy to get. You may want to place an ad in the swap meet.
BTW, a propane BBQ grill works great for heating up the flywheel and ring gear for removal, just be sure not to burn yourself.
