Is all the testing done using the new starter in the YouTube video?
Here one thing you could try, tighten starter mounting bolts, take the negative jumper cable and clip it to one of the end bolts of the starter.
Keep the positive cable on the starter switch nut. Now depress the switch button. See what happens.
If it spins, try the foot operated starter pedal. make sure tranny is in neutral and clutch pedal pushed in. See what happens.
If the above works there is something wrong with engine ground strap connections.
When testing with jumper cables what power source are you using?
When you loosen the mounting bolts and depress the switch button, the starter it spins.
Which makes me think that something might be binding when mounting bolts are tightened.
Did Napa say the starter was a 6 volt starter? (It looks like a 6 volt starter) but you never know for sure after rebuild.
Has this truck been converted to 12 volts? Post pictures of generator and regulator on firewall.
Try this, tighten starter mounting bolts, hook up cables like when truck was new, connect voltmeter to battery, put voltmeter on seat in cab .
Hop in truck and push foot operated starter pedal, at the same time look at the voltmeter and see what the voltage reads.
If the voltage drops close to nothing, the battery is no good.
If battery is good.
Try this, connect voltmeter to starter switch bolt and to one of the end bolts of the starter.
Now depress the switch button, what does the voltage read?
If the voltage drops close to nothing, there might be a cable, cable connection or a binding problem.
