Is the truck now also wired Negative ground?
If it is, the other end of the negative battery cable would be bolted to the engine. If it's somewhere different, where is it? ...and what engine does the truck have?
I'm assuming the truck has a 12v generator, if it's an alternator ignore most everything below.
Here's a link to a 12v '57 wiring diagram you can zoom in on...
http://www.oldcarmanualproject.com/t...re5765-195.jpg
and a 'before' photo of the 12v voltage regulator in a '55 that was previously converted to 12v.
(it's getting a new wiring harness, and a bunch of other stuff)
'A' - Left terminal: the large yellow/black wire goes to the "A" / Armature terminal of the generator, and the small yellow/black stripe wire goes to the 'Gen' light on the dashboard.
'F' - Center terminal: the small black/red wire is connected to the "F" / Field terminal of the generator.
'B' / Battery - Right hand terminal: the mid-size yellow wire goes to the horn relay. The two larger yellow wires... one goes to the 'battery' side of the Starter Solenoid, the other to the Battery terminal of the headlight switch and/or ignition switch.
With some minor differences between years, some wire colors, and how trucks are wired vs cars, this should be really close.
Example: the 3rd image below... a slightly newer diagram with a couple different wire colors but the layout & function is (almost) the same. In image #2 the "+" battery cable is fastened to the left side of the starter solenoid. In the 3rd image the "+" battery cable is connected on the right side. It may seem odd but it makes no difference as long as the large cable to the Starter is on the opposite side.
When you get it all hooked back up you'll need to 'flash' the charging system.
"
.... before you start the engine, remove the field wire from the voltage regulator and strike it (draw an arc) to the battery terminal of the voltage regulator in order to properly polarize the generator then reconnect the field wire to it's position on the voltage regulator." (from this link...)
https://www.ctci.org/gilsgarage/6to12.php
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