Wanted D6 9U 42MT STARTER BENDIX/DRIVE GEAR

Going to finally do it, machine the hole for the electric start on the D6 9U. I have the CW 42MT starter but need the drive. I have studied Old Magnets web sight and read several posts and I think I'm ready! I would like to purchase a starter drive to do the job but torn as to which one 11 or 12 tooth. I was told by Pony Motor Specialists (I think there out of Texas) I could use ether one but should use the 12 tooth. I would think if I use all the measurements I have compiled from ACMOC there should be only one number of tooth drive I should use. I have used this sight before and have gotten what I would say very professional/experienced information. Which drive is preferred (the correct one) where do I find one and how much? Have a happy fathers day. Vic