you know, the 6v starters will handle the intermittent load of being used with 12v quite well.as long as the engine tune is good so they don't crank the starter for extended periods, like more that 10-15 seconds. longer will start to melt the solder in the commutator. is there a 6v starter available that you could borrow or get for a decent price so you could try it? otherwise, my suggestion is to check the web for specs on the diff between the 6 and 12v ring geras and starter drives.
maybe someone else has the answer about the different ring gears? I think it has to do with the number of teeth on the 6v compared to the 12v flywheel. if I remember correctly, and it has been awhile, the 6v ring gear has 139 teeth and the 12v ring gear has 168 teeth? there are different style starter drive gears as well, some with more or less teeth and they don't swap because of different shaft sizes, i think, and also they are a different diameter. this may explain why the starter doesn't seem to work properly.
possibly start a thread with that title? there are some really smart guys on here and someone has probably had the same issue if the thread is titled correctly and they actually look at it.
cruise the net for a thread about ring gear differnces on the chevy 235?
I found this that may be helpful. the second link has a good explanation that may be pertinent to your truck. it may be hard to figure out though because you don't know the complete history on the unit, just know what you have now. anyway, maybe worth checking out.
http://www.stovebolt.com/ubbthreads/...t&Number=11056
http://www.devestechnet.com/Home/Flywheel