Welcome to the Forum Daniel. I can break down your VIN # for you.
It's all on my web site, (www.chuckschevytruckpages.com)
but I'll break it down for you here.
T = GMC (C = Chevrolet)
K = 4wd chassis (C = 2wd)
L = 350 V 8 motor
1 = 1/2 Ton series
4 = pick up (3 = cab and chassis)
9 = 1979 Model Year
1 = Assembly plant GM of Canada Oshawa
423,400th truck built at that plant that year (they start with 100,001)
Ok, I guess I might be alone in my opinion. I can explain. If you were to put money into this truck, you could basically have a "new" truck. Let's be extreme. Say you put $7,000 into it. Do you have any idea how much you can do to a 1979 GM truck with that much money? Provided you do the work yourself, the money goes far. VERY far.
Parts are plentiful, and cheap compared to newer trucks, and other makes. The most useful book you can get is a GM factory service manual. IT has EVERYTHING you want to know. There is a seperate one with wiring diagrams in it. The factory service manual does have some diagrams, of some circuits though.
As far as the NP 203 fulltime 4wd transfer case, it's a beast, and VERY strong. It's rated at 10,000 lbs. GVWR. Not bad considering it's in a 1/2 ton truck. It's a cast iron case, not aluminum like newer trucks. Aside from replacing the chain inside, there's not much more it needs. The chain is about $150. A kit to convert it to part time 4wd is less than $200. Then you'll have locking hubs on the front axle. Changing to a NP 205 is nice, but those cases are getting expensive. Also you'd need an adapter to use it in place of the NP 203. You'd also need a new driveshaft, or one from the truck you got the NP 205 from.
Interestingly, when you convert the case to part time 4wd, you gain an extra gear. The old 4 hi position, becomes 2wd hi. The old 4wd lo position becomes 2wd LO. The old 4 Hi Loc
becomes 4wd hi, and the old 4wd Lo Loc becomes 4wd low.
The 2wd Lo coms in handy for backing trailers up steep boat ramps, or a loaded landscape trailer up a steep driveway.
The rear axle in your truck I believe is a GM 12 bolt. It's a strong axle, not the strongest, but not weak either. It's the GM 10 you have to look out for. It's weak compared to the 12 bolt, and GM 14 bolt.
The body, well, it's rusty I'm sure. THe parts are easy to replace, and readily available. THe cab floor, and rocker panels you'll need a MIG welder to replace. Other than those, everything bolts on. Even the bed is bolted together. About 100 bolts. The beds are the same on 73 - 87 trucks, both GMC and Chevy, both 2wd and 4wd. The hood and fenders are the same on 73 - 80 trucks 2wd, and 4wd, Chevy and GMC. The doors are the same from 77 - 87. The front grille is all that changed during those time frames as far as the nose goes.
It is a good idea to change all the steel lines like you said. It's more time consuming, and torn knuckles than costly. You want to change all the brake lines, including the flex lines out to the front wheels. They go bad internally, even though they look fine on the outside. When you do them, you may need new calipers, and wheel cylinders. The reason is the bleeder valves may be rusted shut. So even if you get the lines out without damage, you might not be ableto bleed the brakes when you are done. Also, when trying to get the brake lines out of them, the lines sometimes want to twist. It's much easier to replace the lines, cyliners, and calipers at the same time.
Replace the fuel lines from the tank to the fuel pump. Be sure to replace the small sections of flex lines too. Add an inline fuel filter while you're at it.
There's another steel line that goes down to the trans module. It's usually ok, because of grease oil and grime on it. At the end of it, right at the module, is a piece of vacuum hose, replace it. Replace all the vacuum lines under the hood too.
As far as the lights go, first clean all the ground connections. I know you said you have no power, but clean all the grounds. Remove the tail lights, and clean the grounds for them on the body. That way you know all your light grounds are good.
The most common cause of headlight trouble on your 79 is the dimmer switch on the floor for the high beams. Rust is the culprit usually. Use a test light at the dimmer switch. If there is only power to the light blue wire there, replace the dimmer switch.
Of course, first you want to make sure you have power at the fuse box for the headlight circuit. Use a test light, and test both sides of the fuse.
To test the headlight switch, use the test light again. If there is power at the red & white wire at the headlight switch, but no power at the light blue wire at the headlight switch, replace the headlight switch.
If there is no power at the red wire, then there's a burned fusable link. This can be hard to locate. If this is the case. I'll look for it on my 80 GMC and help you along.
Well, I gave you an earfull, and can go on for days about these trucks. That's why I made my web site