If there is nothing wrong with the ring gear on the flywheel, I would get a starter that matches it. The 6 volt starter is actually better for the 12v application because it turns faster.
I do not know if ring gears are available or not. That is a big job to be avoided if you can. Pulling the trans, clutch, etc isn't my idea of a good time.
Story Alert! I was working with a very good friend and older gentlemen, John Erb. He is renowned for his knowledge of these trucks in the Central US. He died several years back, but before then, he was Jim Carters GO-TO guy for all AD era parts. He was THE man if there ever was one.
So we get a call from Jim Carters employee responsible for obtaining parts that Jim needed saying he was short on Flywheels. John said.. "are you sure? I just gave you 5 of them" and the kid says "yup needem". So, John and I know all the yards in Ok, KS, NE and MO and we are ready to start our 2 day journey. For two days we pulled flywheels out of trucks in muddy salvage yards with no (or flat) tires buried in the mud. We had a floor jack and some plywood and some rims to push under to support the vehicle.
Understand.. to get a flywheel, you have to pull the trans, clutch, driveshaft, some open driveline, some torque tube. We got to it and got 10 of them out in the two days and of course a trailer full of valuable other parts for Jim. Jim is THE MASTER and a very good friend. He pays fairly, and on time, and he sincerely loves what he does. Whenever John and I were in KC to deliver the goods, Jim took us out to lunch. I have nothing but admiration for the man and his operation. But at the time? DAMMIT!! FLYWHEELS? REALLY??
