You spent plenty of time doing research, just not at the right place!!!
Since your motor is on the stand, do yourself a BIG favor and get me the casting numbers off of the flywheel, and any stamped numbers that may appear. (You may need to look carefully for the stamped numbers!)
You have NOTHING special in your car that wasn't used in other Fords of the 65-69 time frame in Hi-Po Mustangs... You are just not talking to the right sources. I have access to the same books that the Ford parts guys have, perhaps more!
DON'T go buying another flywheel. It probably won't fit in your early style (smaller) diameter bellhousing! If you do, of course then all you will need to fit the new flywheel is a new bellhousing, figure about $2000-3000 for one of those as they are everywhere....!!!
You could also take a picture of the flywheel, thru the starter mounting hole in the block plate and post it here! Might be able to see something that you are missing.
You never did mention how many teeth were on the ring gear? (You never got me any of the other stuff I asked for either, so I can tell you what you have!!! No pictures.....You're leaving us hungry here!!! GREAT opportunity to document a bonafide hipo 289 equipped Goose!!! Perhaps one of the last!)
All you have is a starter problem. Remember? You had all the same parts as before, just that you tossed the starter (nose?) out! Don't go replacing the car just because you lost the key!!!!! We just need to get you on the path to the right starter!
Hang in there and quit messing around!! The experts are here to help you! Just gotta feed us the right info!
You need a starter for a hipo 289 equipped Mustang, 1965-66 is my guess. But get the right info to me and I will be able to confirm this!!!! You have determined that the nose of the auto starter version puts the gear in the right location, but perhaps you need a larger diameter starter nose bendix gear, OR a slight different starter nose.....
Need more info! Call me if you don't understand what I'm rambling about! I'm in the POCA roster.