[1971LT1SWBPOSI373]

Take a look at the wiring diagram, the engine bay side. The prints are listed in the FAQ section above. Wire 12R coming from your alternator/generator and going to the voltage regulator and then to the back of the fuse block also ties in with 18B/W at the bottom of the page. If you follow 18B/W toward the fuse block you'll see a fuse, but I'm most certain that's not a fuse but rather a fusable link. I can't really tell, but it appears 18B/W provides power through the block buss bars for some of the accessories. If this fusable link is blown-it could be your problem.
Another way to start nailing this down is to "ring" wires. Although not particularly fun to do it gets results quickly. Start your truck and take voltage readings coming from the back of the alternator following 12R to the fuse block, voltage reg, and battery. Remember your not looking for 12v (battery voltage) yor looking for 13.5 volt alternator voltage.
If your getting it into the cab, then head for the key switch. Occasionally bad contacts drop the voltage here. Let us know what you find.