I thought at one time someone had a write-up on here where they had successfully installed a painless wiring harness but I can't find it. I do know it is much easier to locate a good used harness if you can. I know of a pdf document that painless put out on the steps of installing one of their harnesses in a 67 Cougar titled "Wire Harness Installation Instructions For Installing: Part #20121 14 Circuit 1967 – 1968 Ford Mustang in a 1967 Mercury Cougar". It may be of some marginal help if you decide to go that way.
As i remember, earlier the Ron Francis wiring harness was way better for us with Ford and Mercury´s when it came to user friendly and easier to adapt to our cars, not sure if the Ron Francis system is still way ahead of Painless tho, others might know that better