Yes, the engine will start, but the teeth will be noisy because of the mismatch of the pitch of the teeth between the starter pinion gear and the flywheel. The most reliable way to determine which flywheel you have is to count the teeth- - - -drop the dust cover off the bottom of the bellhousing, mark one tooth with a felt tip, and start counting as you turn the engine with a pry bar. I usually mark every 10th. tooth in case I lose count along the way! If the starter still has the oval Delco ID tag on it, 6V tags are painted black- - - -12V ones are red. Since inline six engines have a tendency to coast to a stop in one of three positions as a cylinder comes up on compression, it's not unusual to find the flywheel teeth worn in three places due to the starter drive repeatedly engaging those areas. A good welding shop can flip a ring gear over on the flywheel by tapping the ring gear off, heating it to expand it, and reinstalling it on the flywheel with the unworn edge facing toward the starter.
If you've got a 12V flywheel, just install the 12V starter and drive on. There's no difference in the outside diameter of the different ring gears, just a slightly different angle to the pitch of the teeth between the 6V and 12V gears.
Jerry
