I hope you was careful with your old under dash wiring harness when removing it. It is still your best bet.
I get this question all the time.
The Factory wiring is still a lot better.
This is how I repair my under dash wiring. This is a 1967 non tach set up.
This car came to me after the customer work 3 weeks on one of those great American Autowire wire loom kits.
The American Autowire wire loom kits is "NOT" Plug and Play.
These are some before photos of what I found.
It was kind funny how he was telling that he didn't see how he was going to mount his accel pedal.
You have to find a spot for this big ass fuse box.
This is what it look like after I removed the aftermarket wiring. The kit look to have hope on some of the things.
But the problem is where they want you to reuse some of your connectors from the old wiring.
The next photo you will see how not to connect wires. these pull apart with no effort.
The real work was fixing the real nice factory under dash wiring he cut
up to use that Painless wiring type set up.
Here is what I had to work with.
The main thing is solder all connection and use heat shrink.
This is what it should look like before it is taped up.
I clean all wire and connector with lacquer thinner, and then lube the plug with some WD-40.
here is what it looks like all fixed ready to install.