Excess Charging Volts

That is absolutely the cause of your ammeter offset. There is no easy answer and still keep the ammeter. Voltage drop in these girls IS a problem, due to several things....

1...Poor design. I used to service HVAC, including electric furnaces, and they typically use the very same type 1/4" wide blade connectors as the Packard connectors in the bulkhead connector on Mopars. The electric furnace elements are typically "nominal" 20A AND THEY FAIL regularly. So now Ma comes along and tries to use them with 35, 45, 55A alternators. Of course they will fail

2....Too small wire, generally, in the harness

3....If you know where to look, you can find what folks call taxi/police/ fleet wiring, which is where the ammeter wiring is modified (dealer? factory) to feed through the firewall with larger wire than the usual bulkhead terminals. This wiring was used on 65A alternators

You have a choice.......

1...mentally subtract the small amount that the relay causes in ammeter offset

2....do the MAD electrical or similar ammeter bypass and add a voltmeter

3.....do your own custom upgrade, run larger wire through the bulkhead to the ammeter and upsize the charging wiring, etc.

WHEN YOU GET DONE you may still have a drop in the ignition switch / switch connector, and you still might need a relay to prevent the drop.