70 Dart, how to change regular dash to Rallye dash?

The elctrical is the easiest part of it. Move the wires into different style harness connectors, change the oil sender, done.
One can alter the dash sheet metal to work with a rallye inst' panel while in the car. Some dashes are more difficult than others. It has been done before. Cutting bending drilling holes is messy. Welding is to be avoided for obvious reasons. Anyway... One owner stated close to 1200 total spent. He did it right though, reproductions bezel, all instruments restored, etc...
The sale of what you have there would offset the cost a bit.