67 Barracuda Heavy Electric Load

With that higher capacity alternator, also use a larger gage field wire and larger gage ignition wire (feed from key switch to regulator).
Or maybe better, redesign the circuit to use a relay in the engine bay. That will take a load off of the bulkhead connectors, ignition switch, field wires - none of which were intended for the extra loads being discussed (electric fan and alternator with higher field current).

Ignore the MAD advice. While it has some good points, it has a some bad design advice. It's also plain wrong about certain things - one of which is the ammeter seeing 'all current'. It usually sees zero current.

Which gets back to one of the first questions - shouldn't the stock alternator handle the electric fan? Without knowing the alternator, and without knowing the load of the fans, the answer may be no. If the fans draw 2 amps, more likely yes. If 10 amps, more likely no. A plain jane '67 A body came factory with an alternator that could support running the engine and a little more at idle. At 1250 rpm it could run pretty much everything (heater fan, 40 Watt headlights, wipers).

Replacement alternators and A/C cars had slightly higher output but at idle all alternators are at their lowest output.

IF the fans are drawing power from the engine and alternator side of the main circuit, and the ammeter indicates battery discharge, then the alternator is not keeping up.
--------------------------------------
Why adding things like fans to the battery positive can cause damage.
https://www.forabodiesonly.com/mopa...n-1965-ammeters-wiring.517041/post-1973899982
Alternator output and ratings.
https://www.forabodiesonly.com/mopa...with-2-fld-connections.464163/post-1972988945note the factory test assumes 5 amps for ignition and field