Bulkhead wiring/fusible link questions

My plan is to run 8 gauge wire through the firewall into the ammeter as is outlined in the wiring diagram. There is a jumper at the bulkhead connection between ports P and Q. The wiring diagram in the service manual has various notes about with 60 Amp alternator and without 60 Amp alternator, but nothing with about this jumper. If I disconnect the original alternator wiring at the bulkhead, I don't know what will happen with port Q. However, I did just make a practice Packard 56 connection on some wire I have, so I think I'll just replace that alternator connection at the bulkhead and leave it wired as is so that I don't have to worry about the jumper wire.

The car has 12 V when just sitting (nothing on), but as soon as I turn on the headlights or move the key to any position (Run, Accessory) the voltage drops below 11V pretty much everywhere I take it (battery, ballast, alternator field wire, wiper motor), so I'm hoping bypassing/repairing the connections helps with the voltage drop.

Now all that's left is figuring how to connect the 8 gauge wire to the smaller fusible link wire.

Thanks!