if you aready have both wires running out of the bulkhead block you are just fine... the black wire from original underdash harness needs to be there to feed the main splice what spreads the power everywhere, so you should get two wires on black side of amm post. Is a must.

Mostly sure you have also the stock black wire running through the bulkhead block and all allong the engine harness up to the alt( so the same two black wires arriving to alt post )... that's fine too.

aftermarket AC box... wondering where are they sourcing it... I hope not from the bat side of the charging system and correctly from an acc source no matter if using a relay, but from alt side of the game. NOTHING must be sourced from batt side of the game with an ammeter in the line.

The blue 16 gauge fuse link is doing nothing if the other side is not arriving to the ammeter. You can remove it and will be safe of a wire hanging around hot without any function. Tape the red section of the engine side wire on forward lighting harness together with the rest of harness just to keep a clean view ( just a suggestion ). ALTHOUGHT you can hook that wire back to the ammeter as mentioned earlier, just that I would suggest get both red wires on engine bay side spliced together into a just one fuse link. As mentioned, will give you more headroom to the charging system being double wired. Don't need to get worried about the bulkhead conections anymore, they are safe with the parallel path. You have at this moment the same setup I have, also on a 74 Charger and I will correct the fuse link setup too, using just one instead two.

now get a 100 amps alt ( which should be to source up to 55 amps iddling if needed ) and enjoy your setup.