Instrument cluster removal

George
I'm not sure what the back of the cluster looks like for 73, but for the 67-9 Darts, you would need to remove the wiring plugs at the back of the dash to feed the gauges, as well as the speedo cable. I wouldn't think there would be much difference between the 67/9 and the 73 except for apprearance differences.
You should pick up a service manual for your year. It has helped me a great deal over the years.

George