Richard 1928,
Are you saying that the 235 you want to use had an automatic behind it originally?
If so, check to see if the end of the crankshaft has provision for a pilot bushing. I'm not sure they machined all of the cranks for either usage.
Assuming it will take a pilot bushing, then you can reuse everything behind your existing 216 onto the later 235. You can continue to run the 6V starter when you convert over to 12V.
