I have reading a bit more this thread. So far so good I can say:

@jcruse64 you don't need to remove the existant wires from bulkhead if they are in good conditions! You can help them with a parallel wiring. That's why I meant a parallel wiring toward to reinforce the existant one. If they were burnt and you were not interested on reinforce it, well remove could be ONE way to make it. HOWEVER I like the idea about keeping it, why?:

Ok, note the diagram! On a standard setup the main splice on cab side gets tipically a 12 wire running to bulhead and one wire running to ammeter. When you install a paralel wire, the stock wire will be still feeding the splice from alt wire, and the parallel will feed the splice from the ammeter side wiring. So your are feeding the splice on two 12 wires ( no matter the gauge of the wire of the parallel path, just talking about both wires arriving to splice )

If you remove the wire from bulkhead you are limiting the splice to be feeded from just one 12 wire, the one coming from the ammeter side. The only way to make this better is feed the splice straight from the parallel path, and replace the wire between amm and splice with a similar one, instead make it easy just using the amm stud as a juntion and keeping the stock one. This becomes on a deeper modification, going straight to the splice. Is not hard, but reach that?

The extra thickness on the parallel path will help on a more efficient charging process when needed supplying the load to the batt throught the amm if was required.

Next... wiring must be matched to the nominal load the car will be sucking as a constant and able to support some peaks INCLUDING the batt load recharging process. IMHO THIS WAS NEVER CONSIDERED by Chrysler same as the real alternator capacity required to a balanced electrical system.

You were asking about what's the wiring gauge required for the upgrade? Keeping the stock 12 gauge one 10 gauge is more than enough. I used 8 on my car for the parallel path just because I had that wire on hands, but could use very well 10. Use 8 won't hurt anyway.