Another isssue that comes into play is the drive gear on the main shaft. There are two, 8 and 9 tooth. In my 35+ years of rebuilding Muncies, I have seen both used for the same model year application.
I agree with you and Kurt regarding the use of the reducer "The use of the reducer involved a couple of variables such as tire size and transmission shaft/speedo gear as well." (driven/drive gears)
The transmission drive gears you mention 8&9 tooth, was 16 also used?
I have a listing of reducers which shows it was possible to have one on a 373 gear car as well. Have you seen any used on any 15" wheel applications such as z28's and Yenkos?
BTW everyone have a safe and happy New Year