GM 4.3L V-6 Distributor Installation Procedure
In the category of "I just worked on my truck and Oh Noooooo.....It won't start!!"
The gear on the bottom of the shaft is driven by the engine. It spins
the rotor, which delivers the juice that provides the sparks that make
the engine run. Only problem is, engines only run if sparks come at
the exact right moments. The distributor is critical here, because it
“distributes” spark at those right moments. If the gear at the bottom of
the shaft is misaligned by a tooth or two, the rotor’s timing will be off
and the engine won’t run (or at least not very well).

If you were wise enough to seek help before attempting this task, you
were probably told that before you remove the distributor, you should
make a mark on the distributor housing where the metal tip on the
rotor is pointing. That way, when you reinstall the distributor, you
simply line up the metal tip with the mark you made on the distributor
housing, and all will be good when you start up the engine. This
assumes, of course, that you don’t “disturb” the engine, meaning that
the engine does not turn over at all while you’re working on whatever
it is you’re doing that required you to take out the distributor in the
first place.

I never bother with this, mainly because I can never seem to get those
marks to line up properly no matter what I do (or don’t do). Instead,
when it's time to reinstall the distributor, I follow these steps:

Step #1: Remove the spark plug from the #1 cylinder
This plug is on the driver’s side, furthest forward toward the front of
the vehicle. For easier access, you’ll want to remove the air cleaner
and the box on which it sits.

Step #2: Move the piston in the #1 cylinder to Top Dead Center
(TDC)
Here’s where you’ll need either a third hand, or a little ingenuity.
You've removed the spark plug on the #1 cylinder because you need
to move the piston in that cylinder to top dead center (TDC). To do
this, you must turn over the engine by hand, and listen (and feel) for
air being expelled from the spark plug hole. When you don’t hear or
feel any more air coming out of the hole, the piston is at TDC.

To turn over the engine by hand, the Chilton manual says to turn the
engine’s largest pulley with a socket and wrench. This is too hard….
seriously, forget about it (trust me). It’s much easier to put a 24mm
socket on the alternator pulley bolt and turn it
clockwise (don't try
this counterclockwise....trust me again, please). But this won’t work
unless you add tension against the serpentine belt, because on its
own, the belt usually isn't tight enough to turn the main drive pulley.
The belt will just slide over the pulley and the engine won’t turn over.

If you have someone to help, have him or her attach a 3/8” socket
wrench to the square hole on the belt tensioner.  With the socket
wrench set to turn clockwise (tighten), have your helper pull up on the
wrench and give that belt a little more tension. Now, when you turn
the alternator pulley bolt clockwise, the serpentine belt should drive
all pulleys and the engine will turn over.

While you’re using your left hand to turn over the engine with the
alternator pulley, stick the fattest finger on your right hand over the
spark plug hole on the #1 cylinder. As the engine turns, eventually
you’ll feel pressure against your finger and air will expel from the
cylinder. Then, the pressure will suddenly go away, which means the
piston has reached TDC. I usually turn over the engine a few times so
I can get a feel for when the air stops coming out of the cylinder. You
want to be as close as you can to TDC.

If you don’t have a helper to add tension to the serpentine belt, get
yourself a 4” length of metal or PVC pipe (1.5-2.0 inch diameter).
Slide it over the socket wrench that you've attached to the 3/8” square
hole on the belt tensioner. Position the handle of the wrench so that it’
s higher than the battery tray (while providing a decent amount of
tension to the belt). Then slide the pipe back and let it rest on top of
the edge of the battery tray. When you let go, the socket wrench, with
its “extender” pipe, should stay put and keep the belt well tensioned.
IMPORTANT: if you’re using a metal pipe as your wrench extender,
disconnect the negative battery terminal. If the metal pipe makes
contact with the positive battery terminal (which it probably will),
watch out…it will be quite a light show.
Step 4: Reinstall the distributor
Once you've found top dead center, you’re ready to reinstall the
distributor. This step is where it gets interesting. Your goal is to have
the metal tip on the rotor pointing at the “6” mark on the side of the
distributor housing. There is also an “8” mark on the distributor
housing, but that is for 8-cylinder engines. The “6” is what you want.
Because the shaft gears are curved (sort of like a worm gear), the
rotor will rotate slightly as the distributor shaft seats itself. So when
you start dropping the shaft into the hole, you’ll need to position the
metal tip of the rotor at approximately the  “6:00” position on the
distributor housing. As the shaft gear seats itself, the rotor will turn
clockwise to the “7:00” position (approximately) where the “6” is
stamped into the distributor housing. With the shaft gear fully seated,
the metal tip of the rotor needs to be pointed pretty close to the “6”.
At TDC, that mark will align with a similar notch on the engine block.
Depending on how dirty your engine is, the engine block mark may be
difficult to see. Of course, if you’re like the guy in the video and your
harmonic balancer is screwed, this won’t help you find TDC. The
picture above shows the timing marks.
If the shaft isn’t seated all the way down, the metal bracket that
secures the rotor shaft won’t be flush against the engine. This
means the drive gear inside the engine isn't properly aligned with
the distributor gear. If this happens, stick a long flat-blade
screwdriver down the distributor shaft hole and try to locate the
drive gear. It’s about 7 or 8 inches down inside the hole. You won’t
be able to see it…this it totally by feel. Once you locate the slot on
the gear, use the screwdriver to turn the gear. If you’re good, you
can look at the end of the distributor shaft and eyeball the angle
that the drive gear needs to be aligned, so that the distributor gear
will match up correctly. I usually just turn the drive gear a quarter-
turn at a time until I get the right angle and the distributor bracket is
flush with the engine.

From there, reassemble everything and you should be good to go.
Here are a couple more tips and suggestions:

  • The distributor cap, for reasons probably understood only by
    GM engineers, uses a “6-4-2” cylinder order on the passenger
    side (6 being closest to the cab; 2 closest to the front bumper)
    and a “5-1-3” order on the driver's side.  The “5-1-3” is not the
    order of the cylinders on the driver’s side of the engine. As you
    probably already figured out, the “1” is furthest to the front of the
    engine. The distributor cap implies that cylinder “3” is furthest to
    the front, but that is not correct. If you’re using a Chilton
    manual, circa 1998, they didn't get this correct in their
    illustrations. It shows the cylinder order as “5-1-3” in one of its
    diagrams.
  • If you’re really confused at how to stick a screwdriver down a blind
    hole and somehow understand what you’re feeling for, get a
    mechanic’s mirror and a flashlight. With the mirror angled
    properly, you’ll see down the hole.
So...you've found yourself having to do a job under the hood that
required taking out the distributor. Maybe it's your first time, and
(like me) you dropped the distributor back into its hole, buttoned it
all up and were shocked -- SHOCKED! -- that the engine would not
start. Maybe it was then that you decided to pull out the Chilton
manual and found there is a procedure which must be followed in
order for that engine to spring back to life. This is a description of
that procedure, with a few helpful tips and suggestions.

Here is what the distributor looks like, with its cap removed:
Step #3: Confirm that the #1 cylinder is at TDC
If you’re still not sure if the #1 cylinder is at TDC, there are timing
marks on the harmonic balancer and engine block, which will line up.
What’s a harmonic balancer? Well, get underneath your truck and
take a look. Here’s a video that shows the harmonic balancer and its
timing mark:

Harmonic Balancer & Timing Mark Video
Here's how I get a little more tension out of the drive belt.