The answer is simple but rather lengthy. If you know the function of the small wire and the path of the other wires it helps.
The small wire in question, in the stock configuration, assuming no one has replaced it with a plain piece of wire, is a fusible link wire, which protects the secondary wiring circuits in the truck. The primary wiring is just the large cables to the starter and the negative to the frame.
There should be 3 wires on the small junction that you refer to. The fusible link, the power wire to the main junction for all the truck circuits and if you have the gauge dash with the battery gauge AKA (ammeter), a third wire that goes directly to the dash cluster plug to terminal 12.
If you connect the positive battery cable to the solenoid on the starter you will still have to feed the secondary circuit on the fender junction. There are several ways to do that depending on what your plans are for relocating the battery. If you're just going to move the battery to clean up the engine bay then all you have to do is run a 10 gauge wire to the end of the fusible link from the solenoid. If you are upgrading the whole system then you will have to eliminate the fusible link and use a bigger junction to carry the current.
Here's the stock setup.
Here's an example of wiring to a main junction and using two other fusible links in the circuits. This is what the later trucks did after they went to HEI ignition and ditch the ammeter for the voltmeter.