Before we proceed, we need to set the engine to top dead center "TDC". If you don't do this, the truck will not start with the new system and you'll hafta repeat some of the steps again to get your timing right again.
If you don't know how to find TDC, someone else on the forum mentioned the easiest way to do this is to remove spark plug for cylinder #1 (cylinder closest to radiator), wad up a paper towel and stick it into the spark plug hole (make it big enough not to suck back into the cylinder). Once it's in there, spin your cooling fan at a decent speed until the paper towel blows out of the hole. When it does, you should notice the mark on your harmonic balancer should be near "0º" on the timing chain cover on the front of your engine. Line it up as close to zero as possible. Alternately, if you don't mind taking off your valve cover, I guess you could spin the cooling fan until the exhaust and intake valves are both closed for cylinder #1.
Once you're as close as possible to TDC, DO NOT move the fan or turn the engine over.
Take the distributor housing (with the cap OFF!) and insert it into its hole.
IMPORTANT: Make sure the rotor it pointing directly at the plug hole for cylinder #1. In this picture, it's actually pointed towards the passenger side headlight.
You may need to lift the distributor in and out a few times to get this perfect. Once it's pointing the correct way, you can spin the distributor housing freely and the rotor should stay in place while you're spinning it. This will give you a chance to get the vacuum advance and other parts in a good place.
Plug the vacuum advance with a vacuum line cap (or whatever else works).
Loosely bolt in the distributor so it won't jump out, but not so tight that you can't still twist it in circles.