Burned bulk head connection

Hi thanks, I do have service manual and read the Mad article. I also know that the black wire has several connections inside the firewall. Everything looks good there it's just the connection at the bulkhead.
I am planning to run the parallel wiring shown above and mentioned here.
Some considerations about the charging and wiring upgrade and your worries about
Would that make sense and if so, what size of wires?

Thanks Wolfgang