Load is one thing, resistance is another. Smoke requires heat, heat is a function of resistance. I've had a few situations of corrosion with that era of Mopar ammeters. It can be invisible to the eye. I've had essentially invisible corrosion on the ammeter terminals and circuit board bad enough to actually shut the car down as no current could pass (which I suppose is better than having enough current to run but enough resistance and heat to start a fire). Even if they look good, clean those terminals well, an d make sure the electrical transition from the threaded lugs to the shunt is solid. You can test with a charged battery and an old fashion heat resistance battery load tester, on the bench, and see what smokes...