At first I thought you were describing an ignition switch problem, but now it sounds more like a simple power problem.
- If you can't get power to the headlights you have a basic power problem and there is a wire or junction that is flaky.
- If its only a problem with power that runs through the ignition switch, then you may have a bad column switch or the rack and rod is shot. The electrical switch is mounted low on the column where its fairly easy to reach. The ignition lock cylinder has a toothed rack and rod mechanism that connects it to the electrical switch on the lower column. You can reach all the electrical wires for the ignition at the connector on the lower column.
Since you have overall loss of power I would check wire junctions and fusible links.
- First check - On my '75 GMC I used to have random problems with loosening of the connection at the starter where the main battery cable is connected to the truck wiring. This bolt and nut connection is exposed to road dirt and you have to crawl under the truck to check it. However, if that bolt is corroded or the nut loosens up you lose all power because that is where the truck's main power feed connects. Make sure it has a star washer, and maybe an extra lock washer to keep the connection tight.
- Another point to check is the fusible link at terminal post on the firewall next to the brake booster (toward passenger side). The main power wire with the fusible link goes straight down from that junction and you should be able to feel the soft fusible area. It can burn partially away inside the wire without burning the wire completely off. Parts stores sell a replacement GM fusible link in the Dorman or Help! parts area. I believe its a 10 gauge wire with a 14 gauge fusible link.
- There are also two fusible links at the starter post connection. One is in the wire to the alternator and the other is in the power wire for the rest of the truck.
- In addition, the three small wires connected to the starter junction (two wires above, plus solenoid wire) run through a metal tube on the back side of the engine and if this tube is out of place it lets them get too close to the exhaust. They could have fried together or grounded to the tube.