It sounds like your truck's ammeter has an internal shunt.
Because it sounds like it originally had a heavy wire from the fusible link all the way to the ammeter, and a heavy wire from alternator output to a splice or splices with a brank to the other side of the ammeter.
A remote shunt would have small 18 or 20 gage wires going to the meter.

Using that diagram is probably not the best if the rig had an internally shunted ammeter.
From what you've described maybe something like the first one below.
The second one is typical '73 car. I dont have truck diagrams so can't help with that. That motorhome one the guy drew is the only thing I've seen.

15 amps discharge while starting is huge.
The question is why is any current going through the ammeter when you have it wired this way.
As far as the voltage goes, its not very informative. Could be anything from blown diodes to a short sucking power from the alternator.
I think you're best off wiring it up like the factory (using new connectorsand a grommet instead of going through the melted ones).
Then hunt for shorts with battery discnnected, or live, using a circuit breaker where the fusible link(s) goes.