dodge dart rally dash questions

The difference is in how the dash is cut away for the different instrument panels to sit/attach in them. It might be possible to alter your standard dash to accept a rally inst' panel but most do change the entire dash. Hard part of that accessing the 5 bolts hidden under windshield gasket.
The factory rally dash is nice in the cars that came with it but it can be a money pit. In the end that tach is small. Some aftermarket inst' panel might be the better choice.