Wow! The truck cleaned up nicely. You said the truck is now 12v, is the regulator on the firewall just for show? I thought when changing to 12v, the regulator was no longer needed? When I get mine wired for 12v, I'm thinking of putting my regulator on the firewall just for looks.
He may have converted it to use a 12v DC generator, which would require a voltage regulator.
If an alternator was installed, it may still need a voltage regulator if the alternator was for a pre 1967 or so vehicle.