You don't need to change the ampacities of any of the wires whether they are old or new unless you change something that requires a higher amp load. The engineers at Mopar had that pretty well covered in 1966. Age and failure to maintain these old harnesses by the owners is generally what causes their failures. It's really no different than having a P.M. on your main battery cables. Corrosion gets in everywhere. If you don't occasionally clean the alternator terminals, ballast resistor, bulkhead, etc... and especially ground connections the corrosion creates higher resistance and greater load which can destroy even properly sized wire, terminals and connectors. I have personally never had to change to a new harness, but I have heard wonderful stories of ordering aftermarket ones and horror stories about how wrong they were. Best of luck with whatever you decide. The one year only 1966 Barracuda is a true gem especially as it looks like you have the commando 273 4 gear version. Karrrin and I have a Citron Green 66 commando 273 4 gear formula s car saved in our barn to restore when we retire. (if ever)