Bob,
Here are some ideas, I hope they are accurate. Your truck had a generator. If that wiring is still on the firewall, your alternator output could tie into the old regulator BATT wire and it would route it through the ammeter just like it did before.
The '58 has a complicated key switch that has the resistor bypass in it during that START cycle. The schematic I have of a '58 (could be home-drawn) shows only a single wire from key switch to 'S' terminal, and the large 10 gauge or so from ammeter + to large, common lug on the solenoid.
There sometimes is a third wire from the 'R' terminal on the solenoid to the coil side of the ballast resistor, but in your rig, it is usually handled by the key switch, so I can't be certain what the third wire is.
To get net...alternator output usually goes to the - side of the ammeter.
Ammeter + will feed the battery through a large wire to the common lug on the solenoid.
A small wire from keyswitch to 'S' terminal pulls the solenoid in during START.
I don't know how much original wiring your truck has, or if someone has tagged a unfused accessory off the solenoid. If the key switch has been changed, maybe the bypass circuit in the solenoid was being utilized.
I hope I have helped you and not confused you more.
Stuart
