Ammeter Resistance and Voltage drop

I measured that long ago in my Mopars. The voltage drop across the ammeter at full-scale is ~0.7 V. Since you are seeing much more drop, some component may be getting hot. The ammeter bolted connections are a common problem and can melt from corrosion, as can the connectors in the bulkhead terminal (often found melted). The later is not usually an issue in 1963 and 65 cars since they use robust buss-bars to feed the high ALT and BAT wires thru the bulkhead. I like that so much that I installed a 1965 bulkhead (Fury) in my 1964 Valiant.