Replace all wiring?

I replaced the entire engine harness in my 65 Dart, using the lamp harness from a 74 Dart. My engine wires were too hard and hacked up to restore, plus I installed a PDC from a modern car (took a long time).

An after-market engine harness depends on many things, like little support for a slant six. If you are going EFI or electronic ignition, now or future, it changes things. Ditto for a newer alternator or fixing the "ammeter-bulkhead" issue or "weak headlights" issue. After-market harnesses are very expensive and you may have to customize anyway.

I kept my under-dash harness since the wire insulation was still flexible, and just restored it. I removed all prior hacks (many ugly butt splices), added a few wires for new components, added a relay for the accessory switch output, and covered with new loom. Ditto for the body wiring to the trunk. I removed the terminals and pulled the wires thru PVC sheath since the woven cloth cover was falling apart. You could also use plastic loom. While your cluster is out, restore that. I removed the clear lenses and polished, had the face chromed, and used LED bulbs. It was quite ugly before and looks very nice now.