Bulkhead wiring/fusible link questions

That's only OK if the wiring to the distribution point is improved (or the distribution reworked). Otherwise, all it removes is the loads from recharging or running on batteries. Power for everything else will still have to get to the main junction through the firewall. Since the main power wire (black from alternator to P) is in poor condition, with enough resistance, some power will look for a lower resistance route. That will be alternator to starter relay, to fusible link, to bulkhead, to ammeter to main junction.

The most important IMO for this car is to fix, replace or parallel the alternator output wire to main junction. Next would be the same for battery wires and connections to main junction. The most common failures I've personally seen with a-bodies have been in the headlight circuit. A relay arrangement solves that (if when keeping the ammeter, it draws from the alternator output).