I've done this in a 71 Duster. Took all the wiring from a 98 ram with a 5.9 (lights, interior, all of it) and integrated in. The cluster is in place and the speedo works. I had a tone ring laser cut that goes on the pinion in place of the dust cover and made a mount for the sensor. I also used the original heater controls, light switch, but took the column out of the 98 to use that for ignition, etc. Time consuming work, but turned out fairly well. A few bugs to work out, but everything works, including the anti-lock rear brakes. No airbags though. Took the same approach you are, bench test everything, tidy it up, then install in the car.
