Thanks for the status update.You need to create the 'flange' from a ring-shaped section of the front plate of the Yashica body which contains the two brass tabs that allow the lens to slide without turning : reduce it in diameter until it will fit into/onto your C-adapter. You can then drill and tap that 'flange' and screw through from the back of the C-adapter.
Do you reduce it enough so that it fits in the inner section like this?
View attachment 153547
or the wider portion?
10$ + shipping buys you another on evilbay. Keep the one with sentimental value and rip apart a different one....unless you have any thoughts on how to put a C mount onto the Yashica body as well so I can mount some unused lens on it.:smile: