Sounds like you almost had it. The fact that truck didn't start makes sense. Move the wire that goes from the ammeter to the horn to the same terminal on the ammeter as the wire that goes to the B terminal on the regulator. Run a new wire from the same terminal to the B terminal on your ignition switch.
The wire that used to connect to the B terminal on the ignition switch (from the starter) now connects to the ammeter where the horn wire used to be; it should now be the only wire on that terminal.
In summary: one terminal on the ammeter should have only 1 wire connected to it - this wire connects to the starter. The other terminal on the ammeter should have three wires; the new wire to the B terminal on the ignition switch, the wire to the B terminal on the regulator and the wire to the "horn" terminal on the fuse box.
Before starting the engine, turn on your headlamps. If the ammeter reads backwards (shows charge when it should be discharging) just reverse the wires on the ammeter terminals. The ammeter may not move at all as I suspect that the lights may still be bypassing it.
Remember to disconnect the battery before you start moving the wires around.
