Maybe this is a good time to add a thought. It is my opinion that while doing the restoration, it may well be worth the time and money to replace old wiring with new. For our project, I replaced anything underhood. I wanted to make sure it was in good shape to prevent the possibility of fires. We did not have any harnesses with our car, therefore, had to start from scratch.
We got our underdash from a donor car. This worked well to remove and quickly install so we could put everything back without the headache of not connecting something right. However, I should have spent more time on getting the fuse box clean and make good connections before installing. We have not had a problem with wiring, (knock on wood) but can see how some folks would.
Just my thoughts.
Paul