I tried the universal kit and it was not Painless. But, there is a direct plug-in kit that would be a 1 for 1 swap but its around $600. Took me 2 weeks of crossreferencing the painless wiring diagram and the 66 Mustang wiring diagram, horn doesn't work, wipers don't stop at bottom of windshield.
I have bought the kit (20120) for my 66 fastback, haven't installed it yet though. I bought it from Summit and was about $590 USD ($900 AUD including shipping)
If you have an Autozone nearby you can pick it up for about $585 or you can go to their site and get it from there.
From the little that I do know, the stock harness and the new harness aren't interchangable and you have to do a little cutting of the firewall to get the interior loom connector to connect to the engine bay loom connector.
I bought it too but once I saw how "painfull" it was going to be, I returned it. There are better alternatives, there is one that is a direct oem fit with oem connections but I forget the company name.
I noticed their newer 67 harness has alot of preterminated ends compared to the 64.5-66 model harness. I wonder if this is the result of complaints, e-mails, and returns from the earlier harness.
I am just completing the re-wire of my 65 convertible. I found it to be relatively painless but time consuming. The only problems I have had is that it assumes you have an original wiper motor with connector. If you don't, they can't tell you which wires go where. I figured it out by trial and error and a couple blown fuses. Same is true of the heater fan and switch.
Well I am in the middle of an install right now. I have the painless kit for my 66, 20120 or something like that. I do have to say it sometimes is not painless, I am not sure if this is the kit or my inherent lake of experience on wiring a car. The 66 is my first old mustang and my first complete re-wire job on a car. I do recommend getting the original wiring diagrams, I have my downloaded on my pc, it has helped allot having those original documents. Now you do have to attach many wires to new terminals and a good wire stripper and crimper are advised. So far I have the dash wired, yeah that was a P.I.T.A., next time I will wait to install my seats until after I re-wire the dash. I am into the engine bay now and it is proving a whole lot easier then the dash. You will need to order wire holders / clamps to attach the wires to the firewall and fenders, although it does come with zip ties I have not yet found a use for them. It does have a new fuse box, new dimmer switch, new harness fuse that installs just behind the solenoid, and all the required terminals and butt connectors, well unless you are like me and waste as many as you use.
On a side note I am still amazed that you could put your hands up under the dash and potentially get your fingers pinched by moving wiper parts.
I am a "Painless" owner. I have had zero complaints about the kit. As far as it being time consuming, the neater you make it, the more time consuming it becomes. On a scale of 1 to 10, I give it a ten.
By the way, all those wire ties are suppost to be used every 5 or so inches during the installation to keep it neat. Again very time consumming, but a job nicely done .