Please identify this item that sits athwartship behind the engine... muffler? Center lower half of photo, silver cylinder
That's the (raw-water to engine-coolant) heat exchanger.
Is the large gauge painted gold wire the one I should disconnect to avoid fire hazard (ammeter)?
If that's an original Motorola 50A alternator, then yes. Scrape off the gold paint and the wire should be orange underneath. You can verify if this wire still runs to your ammeter.
Remove this wire and bridge an even larger wire (I think I used 4ga) between the alternator output post and the large starter post.

This way, the alternator charges whichever battery(s) are selected on the OFF/1/Both/2 battery switch.
Some people prefer to run this charging wire directly to the house battery bank, which then dummy-proofs it from burning up the alternator regulator if the BATT switch is placed to OFF while the engine is running. If you do this, you then need to decide how to charge the other battery bank(s). I.e., the ALL position of the BATT sw, Automatic Charging Relay, Combiner, or DC-DC charger.