Try these,
http://forum.73-87chevytrucks.com/sm...?topic=11766.0
pick the one that applies and click on it then click on the diagram and the magnifier will enlarge it. The 77 to 80 SBC one seems to show the wiring pretty well but I didn't see one with the key switch in it.
Most of these trucks were wired the same in that,
The large red wire from the alternator goes to the starter and ties into the large cable through a fusible link.
The no. 2 terminal on the alternator has a red wire that also runs to a positive battery connection which is hot anytome the battery is connected.
You can be pretty sure that the no.1 wire a brown wire will go through the firewall and run to the ignition switch. If you have a charging light instead of a battery gauge it may go to the dash and tie into one from the ignition switch. This brown wire will have a resistance somewhere in it which turns on the alternator regulator and protects the diode trio inside the alternator.
The resistance also prevents engine run on after the key is shut off.