I don't like to correct a minor issue but in order to avoid confusion, the device you are calling a
Shunt is just a simple
junction. You will want to use a fusible link for the battery and alternator connections and fuse any loads that exit from the junction.
In our trucks the feed wire that goes to the firewall goes to a four way soldered junction and three of those wires feed the ignition switch the horn relay the fuse panel and the headlight switch. They are all unfused, for some reason the factory didn't find it necessary. the circuits from the fuse panel are fused but the headlights and the ignition are not run through fuses.
If you are intending to use the stock battery gauge, you will have to add two wires to your circuit. I assume you would like to use a voltmeter instead since it is much better indicator of the charging circuit. Also much easier to wire.
Here is a good example of a main junction, sometimes called a distribution block, to give you some ideas and show you that you aren't too far off with your design.
This one shows the battery installed at the rear of the frame but the connections are the same. It doesn't show the large battery cable to the starter solenoid so just assume it's there.
Attachment 1771909
The two gauge cable from the battery is a good idea but it should have been run to the starter solenoid and then the 8 gauge wire from the junction would have been plenty good enough to keep it charged.
IMO the four gauge cable from the alternator is overkill for a 120 amp SI alternator. Again an 8 gauge would have been very adequate.
Here is another diagram with a distribution block that is way yonder better than the old stock wiring the factory gave us. Just keep in mind that what they gave us was quite good enough for the day.
Attachment 1771930
That one also shows the two wires added for the battery gauge. They are the two with the 4 amp fuses in line for the gauge.