Pretty sure I'm not "Shelby or AC correct" but I wanted an old school look so I used a NAPA product called Ashphalt Coated Fabric Loom. Comes in different ID's and is cheap. Example P/N is 737103 which is 3/8" ID.
Here are a couple of pics:
I used different ID's dependent on the number and size of wires I was "looming".
Cheers
Greg