What do I need and what do I don’t!

If you are willing to rewire, I would. If you are not in love with an ammeter, and I do like them, eliminate it. If you want to keep it, make sure the ammeter itself is gone through and solder/ silver braze the studs to the shunt. Upgrade the wiring. If you are going to use a "one wire" alternator, you need considerably larger charge wire. Probably should not use an ammeter with a "one wire."

Certainly eliminate the bulkhead connector. I don't know about the 74, but on my 67 it was not difficult to cut a plate which sits in the bulkhead connector hole and snaps in place with the original hardware. Just cut a hole and large grommet. If you need to pull the harness, it should all go back through the hole

Certainly provide a big solid ground stud so the unibody can be WELL tied to the battery/ engine.