Let me preface: Any time you dig into wiring on these old rigs, you may find DSPO modifications that more or less work but are confounding. If they work, you want them to continue working so keep a record of how they work.
Z&M makes a REALLY decent harness - especially the engine harness. They have the actual connectors where possible, and where impossible ship it with weather/metripaks. It is spendy though. Also - if you get the dash harness - really verify all of the connectors are correct and the fusebox is arranged the same way before starting. I didn't do this and had to modify to work. It probably isn't their fault - I think someone mixed components on my truck.
For my dash harness, I actually just removed it / bagged & tagged / photos of every connector in place in the car — EVERYTHING. Bought a bunch of wire in the common gauges (TSM is awesome to know gauging of the wire). Couldn't find a source for wires with traces so hit them with a paintpen. Also, you can still buy all of the terminals used in the connectors so you have a straight soldered connection, not a splice.
Take the old harness, unravel it (be prepared for your hands to be covered in gooey old duct tape adhesive!), grab a huge piece of cardboard and pin the wires, paying attention to pin where the original harness was bunched / branched. Recreate the original layout with the new wire. Pay attention to the junctions. Release the old terminals from the connector using terminal remover tools. Do things one step at a time.
There were more than a few "what have I done?!?! I've ruined my Jeep!" occasions. Patience, service manual from oljeeps, and circuit tracing brought me back. Was also able to undo some sketchy alterations the PO added with solderless crimp splices, make a "factory fit" for a pile of additional electronics I've added. Was worth it and now know exactly how my truck is wired, drew a schematic of it so I don't accuse myself of being a DSPO (DSCO?) in the future. Took about five weeks of after work effort though.
FWIW - you don't need to remove the dashpad to do the harness - just the cluster. If you are pre '87, have a write up here for removing the cluster:
http://www.forums.ifsja.org/showthread.php?t=182503
Would I do it again? Hell no. I'd trace back and correct / add what needed to be corrected added instead of going thermonuclear.
Too long didn't read: Document what is there in photos, bags and tags. Z&M over race harness. Don't do work you don't need to.