grounding question re: the neg battery cable

Glad you made some headway. Just about anything is suspect in these old cars nowadays, the connectors, switches, and especially the bulkhead connector

These were not exactly wired with oversized wire to start with. Let's say you are towing a trailer --which I did, when I had my old '70RR. Adding another set of tail/ turn lamps to the "already small" tail lamp harness brings a noticable drop in lamp brightness back at the trailer.