You will quickly find, that there is plenty of good help, on this site. Mopar stuff. (It's also in the water, or was that beer?)
Any rate, just the new harness, might not cure all. Know (sure you already have ideas) there are some electrical components, you might also have to re-new. Such as the voltage reducer, found on the backside of the instrument panel. There are also some single wire add-ons, not part of that harness. Same thing under the hood. Questions? Good chance, all you have to do is ask. Car guys, so a dash of BS...grain of salt. Some of these guys (not me), do Mopars in their sleep!
On that 69, pretty sure all of that dash harness fastens to the backside of the dash. Also have wiring, from the harness, to behind both kick panels, that go to the rear.
Like I mentioned in another post, wiring in that shape, is NOT normal. So, keep that in mind, along with what was going on with it? Some kind of problem going on, that created that mess, man made, or otherwise. Fixable? Yep.
Understand your disappoint...good with the bad. Wires only live so long anyway. LOL. Some of mine are older, and still truckin' along, but their in good shape, too.