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

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.

The bolts that hold the dash to the firewall are installed from top down underneath the windshield gasket. You would pull inst' panel,heater case, etc.. to go under there with a di grinder and cut 5 bolt locations.