Toasted wiring harness

[70 Long Fleetside]

Hey group,
I've had my 70 C10 for about 2 1/2 years, and it has run pretty great most of the time, except for an intermittent electrical problem, 2 ignition switches have burned out. The first one burned out right after I got it, I replaced it wondering what would cause that, and drove it for over a year before it melted another one. I of course was worried and should have posted then!
Yesterday I was cruising along and smoke started coming out of my dash, it died, and I opened the hood to smoking wires from the battery to the fuse box, really happy I didn't catch fire.
Anyways I knew I needed to rewire the thing, and now I have to, but does anyone have any idea what would melt the whole harness?
I bought the truck with about 10k on a fresh 400 sb, originally a 6, and have put on about another 10k on it. The guy I bought it from got it to flip and didn't know anything about the work that had been done.
Here's to wrenching, hope I didn't burn up my alternator, starter etc. Time to install the HEI ignition I bought a year ago, haven't looked but the last time I replaced the ignition switch it melted the points and burned out the coil.
It's an original paint no rust barn find that showed 30K on the title and it looked like a 30k body, though there are a few dings. It has 46k miles on it now.
Thanks in advance for any help.