Hi--I was intrigued by your posting since I also have a 348 so I poked around on the web and found this link
https://www.nf6x.net/1999/02/bc-348-faq/--if you scroll down to item 7 of the responses there is some discussion of Williams that might help you along.
My 348Q model is all mint. I was also able to get the shock mount base and the proper connectors for the rear Cinch-Jone type connector. The original dynamotor is still installed and I believe the previous owner kept it in good order and it spins right up on a 28 volt power supply that I salvaged from some old computer gear. I also have an AC supply which I acquired that was made as a conversion unit. Not sure who made it but it seems like it was made as an alternative to the dynamotor in that it has the identical power strip to connect to the leads below the dynamotor baseplate. There are some pictures on my QRZ page and if you look at the ARC-5 rigs, the 348 power supply is on the shelf to the left of the ARC-5 transmitter next to my Lafayette HE-30.
It's on right now and I am listening to a mariachi station here in Tucson, Arizona at the very top of the broadcast band on the 1.5 - 3.5 mHz band. I have it connected to a LS-3 speaker and there is plenty of audio gain to drive it. Not sure why so many hams felt that it needed additional audio but I'm very happy with it as is. The crystal filter is great and with a little care I can still tune sideband signals with the BFO.
The ARC-5 gear was probably never released because they are both in beautiful condition and I use the 28 volt supply to power the dynamotor on the receiver and it works equally well as the 348. They are truly timeless pieces of gear and it's an honor to own them.
Good luck and enjoy, 73, David W2JJV