Well this one should be fun.
Given that you don't know much about the truck's history, we'll have to sort it out as we go along. The one thing we have to have you do first and most importantly is to work safely. I'm sure you are aware that the truck will start in gear if it's a standard shift.I would like you to be certain it's out of gear and the rear wheels are blocked up off the ground before you start troubleshooting .You can also pull the coil wire if it has the points distributor,or the Bat wire if it's been converted to an HEI distributor.
Does it have the original alternator and external regulator or has it been converted to the internally regulated alternator?
Do you have the idiot light dash cluster. or the gauge cluster?
Now you realize that the starter circuits are different between the wiring harnesses, depending on whether the vehicle is a standard shift or an automatic. The standard harness does not have the neutral safety switch wiring, and has no feature to keep the starter from turning if the vehicle is in gear. The later years had a clutch safety switch installed that prevented the starter from engaging unless the clutch pedal was pushed in all the way. I think maybe 74 was the first year.
Do you have a multimeter and know how to use it? It will be really helpful but not absolutely necessary.
Anytime there is a starting problem on the truck where the starter won't crank the engine over, I always like to isolate the starter and solenoid from the rest of the truck wiring.
I do this by jumping the large terminal on the solenoid to the small terminal on the inside of the solenoid next to the engine block where the purple wire is connected. This is called the S terminal. You need a fully charged battery, a good positive cable and good ground connections in order for the starter to turn.
This may create some sparks especially if you short the large cable to a ground, so I like to use a remote starter button to make the connection.It's nothing more than a couple of wires with alligator clips on the ends and a switch to connect them together.
Either way, jump the two terminals and you should get the engine to crank over. If you didn't pull the coil wire the engine may start because the outside terminal on the solenoid will carry 12 volts to the coil during starting even if the key switch is off. Normally the key switch will carry the ignition voltage to the coil after the starter disengages. Jumping it like I described only carries voltage to the coil while the starter is engaged. The outside terminal on the starter with a small yellow(usually) wire accomplishes this task.
So,--- if the starter turns the engine over with this test, we know that the starter and the battery and cable and the grounds are good and the problem is most likely in the ignition switch or the wiring to the starter.
looking at your picture you can see that the wires and the terminal plug appear to be in good condition in fact the wires look new.
Next thing to check is the large red wire in the ignition plug for 12 volts. That's where the multimeter comes in handy. This is the main power wire to the cab harness. It supplies the horn relay, the headlight switch, the fuse panel, and the ignition switch. It should be hot even with the key off, and it should connect to the terminal marked Bat.
Now check the rest of the wires for their correct connections to the key switch plug.
Pink to IGN, Purple to ST., Brown 12 gauge and smaller brown with white stripe to the ACC terminal.
You should have a couple of smaller brown or tan wires connected to GND terminal and if you have the idiot light dash cluster you should have a set of dark green wires connected to another GND terminal. The brown/tan wires are for checking the brake warning light and the green wires are for checking the
temperature light in the idiot light cluster. These wires are not connected to the hot wires in the IGN switch at all and they just ground to the chassis when the key is turned to the start position.
OK to check the starter circuit at the key switch, the easiest way is to jump the large red and Purple wires with the key off and look for the solenoid to engage and the starter to turn the engine over. If it does then we can conclude that the key switch is not making the connection between the two wires and it is bad. If it doesn't turn over then the purple wire is not connected to the starter solenoid.
Here is where the difference between the two harness styles comes into play.
The automatic harness will have a break in the 12 gauge purple wire just above the steering column. This allows the use of the neutral start switch which also combines the reverse back-up lights switch. There should be a dark green and a light green wire in a plug for that. If you don't have the NSS but the wires are there then you will have to jump the two purple wires together to get the starter to work.
If the harness is for a standard shift then the purple wire will go straight to the firewall plug on the inside of the cab. and from the outside of the cab it runs from the plug straight to the starter solenoid. There is a possibility that the connections in the firewall plug are corroded so that the purple wire is not making a connection there and will not carry power to the solenoid.
We'll get into the engine not shutting off after you get the starter problem resolved.
Here are some pictures to help illustrate what I have written.
The ignition switch wires.
The firewall plug
the cab side of the firewall plug
The starter wiring jump the large terminal to the purple wire terminal.