The Harness is not hard to pull out, but before you do, take detailed pictures of how it is routed, where it is attached under the dash, ie plastic clips, etc. Make note of where the ground wires are connected, this is very important, with no ground, you will be chasing problems needlessly. Where the plugs go through the firewall is the hard part. The rubber is 40 years old, and may be hard and brittle, as are the plug connectors. When you start messing with the harness, you run the risk of breaking wires at the plugs. A new harness runs about $400.00 +-, depending on who you go through. The one I got was 100% correct. You have to give them all your options, ie. tilt steering, tach, fog lights, AC, etc. when ordering, as each harness is different. Be carefull of the heater control cables as well, if the clamps are not already broken, they may break really easy. You might want to consider replacing those as well.
Check the wire looms for the taillights and fuel sending units, which go behind both kick panels, these sometimes can be cracked or worn through where they go into the body behind the kick panels. I have found the taillight looms can be melted do to current draw from the extra taillights. I hope this helps and good luck. Mike