Hemi Swapping a perfectly good 70 Duster

Awesome! What firewall connector do you run for the wiring harness?
It's a Deutsch connector, 47 pins. You can use the connector assembler on Wirecare.com to figure out what you need. Sometimes you can find the parts on ebay with better pricing if you search by part number. Wire care is easy because it takes the guesswork out of it. I have these all over my car of various sizes. There's another one in the fender well for all the headlights, horn, radiator fans, etc. Also have it on my fuel tank wiring.
I'll warn you, they aren't cheap. I was first introduced to these when I worked for Cummins. They use these on their industrial engines. I've since seen them in many places such as construction machinery, generators, RVs, etc. You do need a special crimper. There are some cheap options for those as well.
You can see the receptacle side in this picture.

engine comp.jpg