The main page has wiring diagrams in the 'information' section, but they're the same ones you've already seen in your Haynes manual. Here is yours:
http://www.luvtruck.com/PDF/Series6.pdfThis is the main wiring diagram, and Haynes probably included it alone because, although it's dense and nearly incomprehensible, it really should be enough information to get you re-wired....unless perhaps you've already torn all your old wiring out, and need to know exactly where things go in the engine bay. Then the dealer shop manual might be helpful. It's the one with all the detailed pictures and diagrams for each individual system, switch, and component.
It will set you back about 11 bucks on Amazon:
http://www.amazon.com/Shop-Manual-Chevrolet-Luv-Series-351-77/dp/B001GU71YG/ref=sr_1_1?ie=UTF8&qid=1383260358&sr=8-1&keywords=chevrolet+luv+series+6+shop+manualI like to cross-reference as much as is reasonably possible, so apart from the one you've already got, and the dealer service manual, I picked up the 'Chevy Light Duty Truck Service and Overhaul Manual Supplement', although that one is a little more generalized, and written for an experienced mechanic. I don't know if one of those exists for '77 specifically. Didn't notice one during a quick search either.
Good luck