I did this years ago, can't recall which method, but looking at this again, if there is a new charge wire from alternator to relay, why not just take the black that goes from the welded splice to the bulkhead, and plug it into the bulkhead connector where the red wire is now, and eliminate the red/black from the ammeter.
Those contact locations in the firewall bulkhead connector are melted. Even if they were not, eliminating another plug in lowers total circuit resistance which would be a good thing! New wires run through a firewall grommet. Thanks anyway!