In the Chevy trucks of the era, the shunt duty was handled in the factory harness by running a 12 gauge wire straight from the main charging junction direct to the battery 12 volt junction (red in the attached link) and an 18 gauge wire that was essentially the same but additionally routed through the ammeter (charging gauge, black/white at the main charging junction and black at the battery 12 volt junction in the attached link). The gauge is protected by a 1 amp inline fuse right at those junctions. The shunt effect is handled by virtue of the differing wire sizes.
I'd be willing to bet your factory wiring was extremely similar if not exactly the same.
Ammeters do not have any real internal resistance, so if you hook a battery directly across the posts it's basically a dead short to ground. Hopefully you didn't fry the gauge when you tested it.
https://farm4.static.flickr.com/3645...c2a8064d_o.jpg