Go to any auto electric shop and they can do it for you real easily. If you have a 235, anything '55 and up with the same transmission, etc should work well. I stopped pissing around with store-bought starters decades ago and a local shop is the way I roll.
I have not done it with your exact starter, but I have converted both a Model A and a '51 Ford flathead V8 to 12V and I didnt do anything to the starter but hook the 12V to it. It also doesnt matter about the polarity, it will spin the same direction either way. Good Luck!
Rex is correct..........a 6 volt starter that is otherwise in good condition, will operate on 12 volts for years in normal operation. Normal operation meaning that your engine doesn't have other issues and starts readily when cranked. If you prefer, you can have a 6 volt starter converted to 12 volt specs by an electrical shop, as suggested above by bodss396.........it involves, mostly, changing the field coils. Ray
I'm funny about auto electrics. I take the piece in question to the pros, tell them what I want, pay the man what he wants and we're both happy. It never hurts to have a unknown item gone over, unless you enjoy being stuck on a cold and rainy night somewhere.
on the 235 it is most important to match the starter to the flywheel. There are 2 different combinations and you will have engaging issues if incorrect. More specifics here. http://www.jalopyjournal.com/forum/threads/216-235-flywheel-and-starter-differences.5533/
Yep that 6 volt starter will be a "high speed" starter. Just do not use for extended times. You'll like it.
I believe there is a difference in the number of flywheel and starter drive teeth, beginning in'55 with the 12 volt units. The flywheel and starter drives must be used in matched pairs, not mixed. Ray