Fleet Wiring

Your photos didn't load. I am assuming this is the 73 Dart in your signature. My 64 Valiant has ammeter wires run thru grommets in the firewall and that might be factory since I don't see any positions in the 2 bulkhead connectors that would have been used. Also it sure wasn't a taxi or cop car since a convertible. My two 65 cars (A & C) have a dedicated feed-thru lug as part of the bulkhead (screw attachment, not standard 57 spade terminal).

To answer your main question, whether a factory "fleet" or later upgrade, it will work fine in not melting wires or connectors. One question is whether your existing ammeter can handle the increased current of a Magnum alternator (if you keep serpentine belt setup). At a minimum, clean the terminals on the cluster and use silicone grease. I suspect your factory ammeter pegs higher than 30 A since I have never seen one go near full scale.

If really concerned, look up my post on using diodes to shunt any excess current. Even easier is to just wire around the ammeter in the engine bay and use a voltmeter. Since Detroit started skimping on electrical monitors, I use the "voltmeter in cigarette lighter" solutions in my newer cars ("Equis" or such, $15 on Amazon). It is much better than the "car dies with no warning and dash goes crazy" factory solution that bit me a few times.