What is the condition of your original harness?
I have a 1970 car where the engine wires wire cut-off from the under-dash harness at the firewall. I was able to buy a replacement section from Mid-life Harness Restorations in the USA where I can splice it back into the original harness. With a replacement harness running at close to $1k there is a big incentive for me to save the original.
