I knew that wasn't Nacho's diagram because he wouldn't normally show or recommend connecting the alternator to the battery.
That diagram is based on Volvo or something with a hand operated hi/low beam. LOL.

There's at least three different ways to skin the cat. All do the same thing.
The way Crackedback makes the harness, it can be installed and removed with no cutting of original wiring.
Assuming the '76 truck has the same power feed arrangement as the earlier cars, connect to the alternator not the battery.
Looks like Nacho cut in in at or near the hi/low switch.
The way I did mine was to cut in near the original headlight sockets.