From experience...
My suggestion is to get a single wire alternator, wire it directly into the charging system, and then address the dash. I may have some here to disagree with me, but this method cuts out a lot of guess work and is fairly easy to rig. The only problem that I had was continuing to buy the incorrect alternator for the job. User error...
Once the alternator is in place, you can address what's under the dash. Mine was a bird's nest too. Just remember that the red is hot and the purple acts as a daisy chain to connect the gauges.
Good luck!
1970 18 Classic-original
"Beer is living proof that God loves us and wants us to be happy." -Benjamin Franklin