Rewiring an early A-body

So I have a 66 dodge dart with a slant six that I am making look like it is a 65 valiant hand we are having wiring difficulties getting the 66 under hood wiring to plug into the under dash wiring have a 65 Valiant.

Being that the car is 55 years old and the wiring is already quite brutal, I’m thinking that possibly we re-wiring it is the way to go. Is there an easy way to re-wire the car and is there a company out there that makes wiring for early A-bodies? I am also thinking that upgrading the lights on the car to LEDs would be a good idea. Any help would be appreciated.