One further question, also asked by Bob Gaines, is there a part number stamped on the cable, and if so, what is it? The intent of this information is to be able to ask Marti Auto what are they doing.
As the 67 Mustang Electrical Assembly Manual calls out C7DF-14301-A for all engine applications, 200CID I6, 289CID V8 and 390CID V8, with the same routing, along the outside of the battery, I asked a friend about his GT500 and he says it's routed the same.
I also reviewed the Ford Car Parts (aka MPC), both the 60-68 edition and the 75 edition, and there is a service part number change, but the length is shown as 41 (or 42) inches long.
Jim