Welcome to MM Forums....
Were do I start.
Everyone here that knows me, knows how I feel about aftermarket wiring kits.
The idea is great, new modern wiring to handle all the wonderful items that we like to install in our modded updated rides. The problem I have with them is that they are pretty much spools of wire with printing on them with no connectors to anything. I think that the aftermarket kits are in there infancy right now. Few years from now they will actually sell a good kit that is made with the main connector you need to make installation easy a great improvement to what we have to chose from now.
I wire the classic Mustangs Every week here at my shop. In my area I am now as the Classic Mustang wiring Guru. I have see it all, and install every toy that you could think of in a classic Mustang. I seen the Painless kit, The American Autowire kit. and they are not worth the money spent of them.
The best way to go still is the Factory wiring. It has place for everything you need to make all the toys work in your car.
The under dash wiring is what I try to save, It is usually in real good shape. It is not exposed to the weather and you have to fix the spots were the kids try to add the stereos. Then I buy the new head light side and engine gauge feeds, & tai light wiring.
It all plugs together in 1 hour and ready to go.
here is a link to me cover this same subject. https://www.moddedmustangs.com/forums...gs-wiring.html
Let me know If I can be anymore help, Be sure to look at my cars I have built. See if I can give you any ideas or you can give me a few ideas.