A bit more follow-up. When I measured these wires, as best as I can remember, I measured from the end of the wires sticking out of the distributor cap boot to where the wire ended inside the spark plug boot. So, for overall length, one-half inch could be added for including the length of the spark plug boot.
All the boots on these wires were straight. On the 1964 Durability Comet that was in Harrah's Auto Museum at the time I examined it, the HiPo wires all had straight boots. When I looked at some NOS sets of HiPo wires, all had a couple angled boots. So, I believe that there was a difference between production HiPo wire sets and service replacement (NOS) sets. If you have a Mustang HiPo with original wires (not NOS service replacements), it would be good to document the length and type boots (angled or straight) that were used. In 1963 and 1964, I believe all original HiPo wires (excluding Ford service replacements) had straight boots. I don't have any evidence for 1965 and later.
Also, the routing of the wires generally followed the following rules:
Passenger's side wires went through both wire separators in a 1,2,3,4 order.
Driver's side wires went through both separators in a 7,5,6,8 order. #7 then went over 5,6 on its way to the spark plug. This routing is supported by vintage magazine pictures, the 1964 & 1965 engine assembly manuals, and my observation of low mileage originals.