1979 just shutoff. Everything dead but the lights

[my240]

Don't want this to be too long but need to explain. I have a 1979 k10, 4x4,auto,400 sbc, shortbed ,all original with 51,000 on it. Truck has been flawless for 40 years. Still have the original uniroyal landtrac spare. A few weeks ago I was coming into driveway and it shutdown. No crank, everything dead but the headlights. I pushed it into garage and started changing things. New battery, alternator, starter (checked for frayed wires), new key cylinder( old key had no teeth left on it), new lower ignition switch, checked fuses in fuse box. It fired up and ran like a new one. Has done good for two weeks or so. Today I ran to town and when I came back to the house it shutdown again in the driveway exactly as before. Everything dead when you turn the key no fan, dash lights, nothing. I've changed all the big items but I must have missed something that must be shorting out and accidentally fixed it before. Give me you thought on where to start this time around. I really hate the thoughts of a completely new wire harness. Appreciate any ideas and sorry for length of this thread.