I took a slightly different approach on mine...but, then mine had had a fire behind the dash sometime before I had bought it which compounded the problem.
I ran an 8 ga wire from the alternator to the starter relay. I then ran another 8 ga from the relay, thru a grommet I installed in the firewall and onward to a terminal box I mounted on the end of the hvac box. That gave me an underdash source of bat power to connect the circuits that orignally came of the alternator post.
Being that there were no longer any fusible links on the car after some prior owner had rigged it, I also made sure that everything was fused just in case.
10 ga would have been enuf, but, I had some flexible 8 ga on hand. As others have said, the bulkhead connector really needs to be cleaned up as there seems to be a lot of resistance in the connections there. Using relays for the lights, and ignition, would seem to be a good idea there, as well.