My 67 Dart has a 4.10 diff so it pulls pretty hard from a stop as is! That car could run a 518 with that axle gear no problem.
The Charger currently has 3.91s but I have other ratios here including a 3.23. That would make a sweet freeway cruiser with the 2.23 final drive.
I have a 2.27 final drive in the red car with the Tremec.