Need advice for diff for 65 Dart factory hi-po 273.

CudaFactHackJob said
How do they make an A833 into an overdrive?

It is a whole gear swap. 3rd gear was the O/D and 4th stays the same at 1:1. That is why you need to flip the 3-4 lever upside down. Otherwise 3rd would be a higher gear than 4th.