^ I'm at home (briefly), was able to check out Caltrans' old paper logs as well as the more current online Postmile Query Tool. Both put CA 76's west end at I-5 rather than Coast Hwy (US 101 Historic), with CA 76's postmile 0.0 in the I-5 median. So I'm leaving CA 76 as is in the Highway Browser, even with the otherwise convincing signage shown in your GMSV links.
I think a labeled waypoint at the Coast Hwy./Harbor Dr. intersection (HarbDr_W, since there's an intersection with another Harbor Dr. in San Diego) best reflects the end of signage for the historic route, and the approximate north end of the old pavement (obliterated by the new I-5, until it resumes within the Marine Corps base). That also more clearly shows the historic route as separate from the parallel I-5, even with the moved and renamed I-5(54A) point on the south side of the river.
No signage on Harbor Dr. between Coast Hwy. and I-5, so I'm not as sure about including the I-5(54C) point as the historic route's north end just to maintain a graph connection with I-5 - @si404, any thoughts on that, as (I assume) the original drafter of the route file?