Some of these old trucks would burn the wires off the back of the ignition switch, they use to make a replacement plug to replace the bad one. This truck may or may not have fuseable links , usually located at the starter solenoid. If you have power coming to the F terminal of the alternator and aren't getting out at least 13.5 volts at the bat terminal of the alternator . This usually points to the aternator itself. If there is no power to the field (F} terminal this points to the regulator. Most of the electronic regulators have a wire that has power when the ign switch is on , one that has 12v from the battery for sensing voltage, and the {F} wire going the the field terminal of the alternator voltage varies with output demands. To check the alternator a hot jumper wire connected to the field terminal will let the alternator produce full output.

Have A Great Day --- Hotwrench