From everything that you have told me the truck should start.
You are getting the test light to come on when using the key and testing the trigger wire to the starter solenoid, Correct?
If you jump power to the starter, the engine turns over, Correct?
If you jump power to the trigger lead on the starter solenoid, the engine turns over, Correct.
Right now, with everything hooked up, the truck should start.
The only thing that I can come up with is that there is a lot of resistance in the circuit from the key to the solenoid and you are just not getting enough current to the solenoid for it to work.
We may be back at the fuse box and some corrosion there. Normally that causes a lot of other problems, like the alarm sounding, the radio not going off, the wipers working on their own, interior lights doing stupid things, etc.
If you turn on the key and jump from the bottom lug to the trigger connection I am betting that the truck fires right up.