The intake manifold is two pieces. The large aluminum casting, and the steel plate that seals the bottom of the inside of the keg. That gasket likes to blow out, giving you a nice vacuum leak, and also leading to increased oil consumption. You have to look down inside the intake to check it. You can try just opening the throttle all the way, but, pulling the throttle body makes it much easier to see down in there. (flashlight is a requirement as well.)

Distributor cap sits on top of the distributor. All the plug wires connect to it. The rotor is underneath the cap. Take off the cap, and there it is. They are replace generally together, and you don't have to replace the entire distributor to do it. You DO need to pay attention to the order the wires are place on the cap though. Cylinder 1 is marked, then, just follow the firing order. (clockwise.....)

 

These might help

 

Why does the arrow point clockwise on one, but the order isn't 1-8?

 

Huh? They are both the same, just one is rotated 90 degrees.

 

I don't know why firing order is so important on distributor, but
I know when I plugged in my wires out of order the truck runs like crap.

What's with the clockwise part on the diagram?

Why clockwise if order isn't in sequence?

 

I suggest you do some reading on internal combustion engine basics.

 

When in doubt, YouTube is your friend.
Research key words:
Timing
Internal combustion engine basics
​​​​​Distributor
​​​​​​How to do a tune up

 

Just for fun... Mainly a writing exercise for my head but it might give some young people a step up.
Sometimes a key piece of information is not presented in a manner which informs someone of a basic idea of understanding a concept that they can build upon. I totally sucked at algebra until I met the right professor.
Engines spin at mind boggling speeds and its hard to believe that they don't all just fly apart. Put your foot into it and each piston reaches a maximum speed of nearly 50 miles per hour in mid stroke and comes to a complete stop, reverses direction, and then gets up to nearly 50 mph in the opposite direction only to totally stop again after traveling only a few inches. Contemplate the acceleration and deceleration. Zero to 50 MPH back to zero twice in under 2 tenths of a second. Its not the speed but the acceleration and deceleration of the piston that is mind boggling. There's an awful lot of metal mass going up and down and round and round.
An engine with one piston is going to shake a lot. With two pistons, consider that you get one power stroke for every two revolutions, so both pistons go up and down at the same time but you alternate the power strokes to smooth out the engine output. With four pistons, two pistons are up while two pistons are down resulting in an even distribution of two power strokes for every revolution. Remember all that mass of each in piston going up and down? If you were to put the firing order of the pistons in number sequence, you'd create a wave of forward to back engine rocking motion. It could become so severe that it might eventually bend the crankshaft over time not to mention make the car shake. So designers typically stagger the firing order to something like 1342 on a four-cylinder engine. Pistons 1 and 4 are both up or down at the same time and pistons 2 and 3 are up and down at the same time. With Piston 1 on the compression stroke, 2 is on the power stroke, 3 is on the intake stroke, and 4 is on the exhaust stroke. This is worth wrapping your head around.
With an eight cylinder engine, designers alternate and distribute both which pistons go up and down at the same time and the firing order in which they deliver power. On your truck, Pistons 1 and 6, 8 and 5, 4 and 7, 3 and 2, are paired to be at the top of their respective cylinders at the same time. They fire off in an order that is designed to maximize the smoothness of power output while minimizing vibration. Its a big deal. The more an engine shakes, the sooner things will fall apart from metal fatigue.

The other half of this is the distributor. As folks have mentioned basic combustion engine theory, understand that the cam shaft for the operation of the valves spins at half the speed of the crankshaft. The distributor rotates with the camshaft at exactly half the speed of the crankshaft. This relationship of crankshaft to camshaft and distributor is permanently fixed by gears, a chain, or a toothed belt. After "billions" of revolutions and years, an engine that is still running will never once encounter a situation that allows the relationship to vary in the slightest. All eight cylinders will need to fire for every two revolutions of the crank. On your truck, the ignition coil will generate a spark eight times for every two revolutions. Inside the distributor is a rotor that creates a path from the center connection from the ignition coil to the tip of the rotor and the closest spark plug wire to the tip of the rotor will take the coil voltage to that spark plug because electricity will follow the path of least resistance. The distributor does exactly what it's name implies, it distributes the high voltage pulse created at the ignition coil and it does this at exactly the right time when the piston is in a place to begin a power stroke. Its in a way the rough timing of the ignition. The ECM/PCM or computer does the fine adjustment of the ignition timing.
Apologies to those who are annoyed.