One thing [I]'m curious about is how the ammeter wires are wired on the battery and alternator end. The 12 wire should start near the positive battery terminal and the B/W no.1 wire should start at the main junction of the truck harness, with a shunt wire between the two points. The length and size of the shunt wire is also a factor in the accuracy of the ammeter reading.
Here is the original factory wiring with the external regulator in place.
Notice the battery and alternator are wired on the ends of the shunt wire.
When converting alternators or going to a one-wire alternator These rules still apply.