The California Official map for 1964 can be found here:
https://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~239521~5511847It (and previous years) shows the actual US-40 coming into town from the north on a one-way pair consisting of 12th St and 16th St, with the southbound direction jogging on F St to 15th St. These connect to an apparent two-way N St, which uses a one-way pair on 7th St and 8th St to get back to M St, and out the west side of town. N St is now one way, so the obvious currently drivable route is L St to 7th St to M St. I drove that a year or so ago, but drove it in the other direction too.
The last two years in which US-40 is shown on the official maps are 1965 and 1966:
https://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~239527~551185115th St and 16th St came down further to another one-way pair on P St and Q St. Then, another one-way pair on 3rd St and 5th St went back up to M St, whence it left town.
Whether anyone ever put up Historic US-40 signs on any of these routes, I don't know. I don't recall seeing any such signs anywhere in Sacramento when I drove around looking for that road, and I still don't see any "driving around" in GSV. So TM could remove it entirely from Sacramento, or use the cleaner 65/66 alignment, or the more "historic" 65 and before route. Is TM trying to show the actual signage on the ground, or the actual history?