With the alternator you chose, all you need to do is break the ignition feed to the alternator, and you can do that with a smaller relay, and by using a 4 post disconnect

A 4 post disconnect has two large posts for the main battery feed, and two smaller ones.

You could, for example, put a general purpose relay up front near the regulator. Break the ign feed lead to the regulator, and put the relay Normally Open (NO) contacts in series.

Run two no 18's or 16's back with your cable to the trunk, hooked to the small posts on the solenoid.

Wire like this:

Starting at the point where you broke the ign feed to the regulator, hook one wire to the IGN side coming out of the bulkhead (ign run)

the wire goes back to the disconnect and back up front. Hook the other wire coming up front to your relay coil, and ground the coil.

In other words, with key on and disconnect on, the relay closes and feeds the regulator. If you either turn off the key OR the disconnect, the alternator stops charging.

Another way to do this would be to run two no 10's back to the disconnect (they may not need to be quite this big, but hey) and eliminate the relay.

In this case, hook the no 10s coming back to the small posts, and up front, once again, break the IGN RUN feed to the regulator. Hook the two no10s, one to each end of this break, and you're done.

(The reason you need either a relay or large wire for this to the rear, is this IS your battery voltage sensing lead, and now it's a "lot longer" than it used to be)