You have to evaluate the interior and external harnesses separately. The interior should be fine providing no previous fire from a short or no butchery (radio swaps, old CB radio, etc). All that may need to refresh is a rewrap with the correct tape unless the original can be cleaned up. Try to preserve any tags if they exist.
The engine harness is exposed to a lot of heat and may get some brittle wires especially on the harness section that runs to the starter motor. If that is too brittle then you can get a repo for that one section. The ignition wire also needs to be checked closely.
I still have my original harnesses on my 67 but I did change a few burnt wires from a donor harness so if you can find a donor you can just replace what needs replacing.
The trunk harness is usually in good shape providing no external damage. I would try to keep as much of the original harness as possible providing it can be safely done.
How does the fuse box look as far as corrosion?