IMO, it is better to replace than try and restore 40 yr old wiring. It will/does degrade with time.
As previously noted, the repro harnesses are very high quality and I would strongly recommend using them.
I made the mistake of tyring to restore the front light and engine harnesses on my '66 Chevelle back in 1997 only to replace both of them in the last 3 years due to ground issues. On the Chevelle it isn't a that big of a problem. You can easily reach down to the bulkhead connector and individually release each harnesses at the bulkhead connector.
By the time I was reworking the front end of my '69 RS in 2004-05, I fortunately had learned my lesson and replaced the old harnesses with new ones while I had the front of the car dissassembled. Once everything is back together, at least on my car, it would be very difficult if not impossible to access the bulkhead connector, unbolt both harnesses as an assembly and then separate the harnesses without revmoving other items first; for example the power brake booster and master cylinder would have to go at a minimum.