I would restore the under dash portion. There is usually not much wrong with them.
Marty
+1 Besides, on just the dash harness, there are a few running changes and I believe a bit of changes between original suppliers across the span of production, and not just the "with tach" or "without tach" options. Headlamp harnesses (if I have this correct) are also "with tach" or "without" Then add the "with Fog" and "without fog" and mix it with the "with tach", "no Fog" and so on. Alternator harnesses are 6 cylinder, Big Block or Small Block and the with tach and w/o tach also applies. Engine feed is the same "with or without tach" (though I cannot recall seeing a 6cylinder with tach option) for the 3 basic engine blocks.
Tailamp harnesses had a few options too, "With Low Fuel" and "without", and I believe convertible was different than Coupe/Fastback. So a few variations available there too.
I would try to restore all but with that said the taillight harness and headlight harness are the typical candidates for repro replacement. The headlight harness unless substantially cut and spliced is relatively easy to restore. The taillight harness with the weave covering not so much.
Headlamp and engine harnesses take a lot of heat over the years and the plastic insulation gets stiff and begins to break down. The weave covering is often ratty looking on the tailamp harnesses and then the taillight sockets look rather poor for aging (or rust).
Alloy Metals is said to be the best repro available at this time. NPD and Virginia Classic, as well as many other good Mustang retailers sell them. Mr. Mustang is a cheaper version, but not much cheaper and said to not have as good of quality. It has been said in various discussions I have read that Scott Drake uses the Mr. Mustang version of the reproductions.
I have read info for a few years at different sites, mostly for my door harnesses and the tailamp harness, so this information is based upon the efforts of my searching.
I am in the middle of restoring mine and I was surprised my headlamp and alternator harnesses are rather nice considering they spent the first 30 years in HOT dessert areas of So.Ca. I will be probably sending them out to Randy here on the forum (username Midlife) to go over and repair/clean them up. Nothing more frustrating than "electrical problems" on a fresh restore, and nothing stands out as ugly as old wiring flapping around the engine compartment and trunk.
Richard