1970 Chrysler 300 - No Power

I had very similar issue on 67 cuda. 273. Elec ign and mostly clean original harness. Altho it is getting brittle/cracking. I would check the ign switch itself also. Is it in dash (hopefully).
Not sure if abodies are wired the same as yours. Large gauge black(I think) wire goes to bulkhead connector may need to be replaced/upgraded. Check bulkhead for melted wires/plastic.
Good luck. Beautiful car.