Slant six over heated and now will not keep an idle.

AND! Make sure to clean all those contacts on the bulkhead connectors. Old Mopars are notorious for issues here. I'd take a light copper brush and gently remove any corrosion on both sides. Then take some dielectric grease to keep dust and moisture out. That big fat red wire was probably the wire going to your ammeter. On stock harnesses it goes through the bulkhead connector with a fusible link to protect the wiring. The fusible link is designed to blow before anything else. They make new ones too! Perhaps all you may need to replace is that wire. Personally I like to make sure everything is clean and stock to everything else under the hood. Best of luck.