let's see, "high dollar cars" ( mostly of the time being show cars ) don't have extra added equipments incorrectly sourced from batt post which is one of the most common mistakes ever made on our cars, neither have been driven enough. Aside this, the next big mistake is replace the battery with a higher reserve load than the stock one keeping the same alternator. This multiplies the load charge time on the battery ( with an uneficient alt ) adding more stress on the charging circuit for longer periods.
about your worries.
1- doesn't make sense get an 8 wire on red side being the stock one between starter relay and battery still a 10 gauge wire ( and of course 16 gauge fuse link )
2- with an alt upgraded ( AND THIS IS THE MOST IMPORTANT THINK, YOU NEED A GOOD ALTERNATOR TO KEEP SAFE THE CHARGING SYSTEM AND THIS INCLUDES THE WIRING AND TERMINALS ), your battery wire ( red line ) won't get the same stress than before the upgrade because the battery will barelly be requested for loads, so once again, you don't need an 8 gauge wire for the red line.
3- we can see on your old bulkhead the bigger stress is on the black wire cavity. This is because along the years not only suffered the load requested by all the car circuits but also the load requested when batt was discharged, while the red wire just "suffers" the charge status when is discharged, hence the reason the red wire survives better to all these years, mostly sure because your car didn't have added accesories on batt side... or just one ( i.e. headlights relay ). So another reason why you won't have to worry about get the red wire upgraded to 8, while is enough upgrade it from the 12 stock wire gauge to 10 and removing the packard terminal which is actually the WEAKEST point, not really the wire.
I simply suggest wat I will make, even more with a new fresh/renewed/mantenienced installation made... and don't over think more this.
the reason why I suggest this is:
1-the main splice will be getting power from both sides, bulkhead leg, and ammeter leg. This makes a better source for the splice not just getting the ammeter ledge in just one 12 gauge wire, but two of them ( 12 gauge ) as indicated on graphic.
2-Using still the existant and fixed/mantenienced 12 wire between alt and splice, there is no reason to have a 8 wire on the bulkhead bypass path. You will be using 10+12 gauge wires which is the same than use one single 8 gauge wire.
3-I removed on diagram ( and will make the same on my car when reassembling ) the stock red wire on engine bay, because:
3a-simplicity using JUST ONE FUSE LINK from my previous setup
3b-is SAFER just use ONE FUSE LINK, because dual will take longer to get blown while the failure burns whatever gets to burn
3c-As mentioned, with an upgraded alt this wire barelly will see any load for long time, or high peaks.
Now, I know 72RoadrunnerGTX is not convinced about keep the stock wire because is still with the packard terminal on the line and I can understand it, however I can testify my car has beeng running like this FOOOOOR YEEEEEEARS, being a TRULLY DRIVER CAR, EVERYDAY ALL DAY LONG... AND NIGHTS, And I have not just two but 6 RELAYS installed down the dash frame, because I USED THE A/C too daily. And I added a relay to each speed, aside the headlight relays. ALL RELAYS are being sourced from ammeter black stud. Of course the max relay sucking power will be 2 althought it could get 3 activated ( but still just two sucking loads )
This has been my setup for YEARS, and the only difference with my next step to be build will be:
1- not anymore stock red wire on engine bay and double fuse link
2- will be using a molded quick disconect plug... however, I INSTALLED on the upgrade lines regular bullet kind terminals sealed with shrinking tubes because I like VERSATILITY. I like get the chance to disconect any wire anytime just like Mopar did.
These are the terminals I used to be able to disconecty the parallel paths on engine bay side ( on red wire is the one I used to link it with fuse link, which would be the disconect point on red line ), but mainly because these terminals are what I could find in Venezuela.
and now will be using this, because it gets a better look ( more professional and "automotive stockish" look )
The quick release loop I got is 2 feet long which should be enough to cut one long side to reach the ammeter from engine bay side without splices, and use the short side to splice the rest of wiring on engine bay. Red splice will be mostly sure innecesary on my car because I can use that cut to attach the fuse link with bullet terminals, but my 74 gets the starter relay just right above the brake booster, somehow close to bulkhead. 71/72 get the relay at a side of the batt, so splice the red wire to reach starter relay will be mandatory.
ASIDE THIS, I used 8 gauge wire on the black path on my car but just because I had that wire already in hands from a donor 76/78 B body full harness setup ( which uses a shunt system ) I got from a JY, but as mentioned, will change for a 10 gauge and I really trust on that.
I began using a 60 ams alt, then got a 78 amps alternator but now I'm running this ( with smaller pulley which I changed ), which I can't tell is really a 100 amps alt ( I don't think Chrysler ever made these on that rate, still with the box and decals ), but it works like a champ, even I still want more at iddle and will upgrade someday to a TuffStuff or PowerMaster trully 100 amps able to give 65 iddling
AAAAAAND TRUST ME... my car before get dissasembled, has been a DRIVER CAR DAY AFTER DAY ALL DAY LONG ( with its nights ) on a dense traffic city for 6 or 7 years with the second diagram wiring job I posted on this reply. I had a girfriend on another city and I drove it to his home weekly, and traveled on vacations to some other parts in the country with it ( and her )... running A/C, at nights, down the rain ( wipers sucks A LOT, just like the A/C blower )!!! etc...
