You're exactly correct. Due to the age of these cars we should expect some strange problems to arise.
The Gauge Doc got back to me and my speedometer is repaired and it is on it's way home. Jamie Passon also got back to me and he told me for the Hemi OD transmission the output shaft does indeed have 13 teeth just like a regular 833 4 speed. So the 38 tooth gear I have in there (in theory) should be accurate once I get the speedometer installed. Dean from the Gauge Doc asked me if I lubricated the speedometer cable when I installed it? I did not know about this so I intend to lubricate it as soon as possible. There's always something new to learn about these cars!