Remove complete dash?

You have to remove the windshield. After that, it's pretty straightforward. There are 5 3/8 head bolts along the top of the dash that are exposed once the lower windshield gasket is removed.

It is also helpful to remove the 3 bolts that hold the steering column to the floorboard AND the three bolts that hold the steering column to the dash. You don't have to remove the steering column, it will drop down out of the way.

Remove the kick panels and there is one 7/16 head bolt on each side holding the "legs" of the dash frame. With these 7 bolts out, the dash will fall into the seat area.

There is one 1/2 head nut that holds the parking brake lever to the dash which can now be removed easily from the top. You will also need to remove the heater control cables or vacuum lines, the defrost vent hoses, and unplug any wiring harness plugs to the body.(brake light switch,radio speaker,door jam light switches, harness to tail lights,) What's left of the firewall connector can be removed too.

After all that is loose, you can work the wiring harness on a workbench. I have a 71 Duster ac parts car that I'm parting out. I haven't checked the entire harness but I do sell them when I have good ones.

Removing the seat is a good option too. If you remove the gauge cluster you will have a lot of room to work.

Good Luck!