What do I need for a proper under-dash rewire?

The wiring harness will come out without pulling the entire dash.
If you'll separate the bulkhead connectors outside and shove that box into the cabin, drop the light switch and the wiper switch at bezel nuts, same with flasher modules, bring those out still connected, 1 screw in the fuse box, a connector behind the right kick panel, Pull the instrument panel, get the harnesses branches out from under the very few straps...
Yeah there's a brake lamp switch connector, reverse lamp connector, blower switch connector, and another at blower resistor pack, glove box lamp, wire to right door jamb switch.
If you think that reads like a LOT, remember its nothing compared to later models.

Would it be easier to remove the dash for access to replace the entire harness? Can you do that without removing the windshield? My seats are out so there is room to move around but I will be replacing mine soon on a 73 scamp.