alternator output
It's likely your battery is down a ways and charging that hard, but ^^above is right, the factory ammeter and wiring were NEVER designed for that kind of output Get started by reading the MAD article. Some guys don't like the way that is done but it IS a very good overview of the problem and why.
Catalog
Also do a search on "fleet wiring" which is heavier ammeter wiring on 65A optional cars
See this thread
68 Dart Voltage Reg "issues" - Where is factory mounting location
mopar "fleet wiring" - Google Search
Here's a shot out of the '72 shop manual this is B body not A body but shows the fleet wiring option
Here's an old thread on ammeter to voltmeter conversion. Pleas read it, as it covers a lot of ground
Ammeter to Voltmeter...who does it?