Help! Converting an AC car to non AC wiring question

I am converting an AC car to non AC car. The question I have is, The section of the wiring harness that connected to the old heater box and control panel is not the same as my current heater box and panel. What do I need to do As far as getting the wiring corrected? Is the section of the harness I need available? Do I need to replace complete under dash harness?
Vehicle 74 duster was /6 AC and now is a 318 non AC car.
Any help or direction would be greatly appreciated.
Thanks
Dan