I recently tried this with my P-bass, which has a '62 Jazz RI neck. It didn't work, but that's because I'm a technical moron. Other people have done this operation with success. (Read about it
here.)
I bought some of the standard #8 and #10 size washers, and they were both too big in diameter to fit down the hole. When I went down a size to #6, the hole in the middle wasn't big enough to fit over the truss rod. There doesn't appear to be a standard size that will fit without a little bit of extra work. If you read the thread I mentioned above, people talk about drilling out the hole in certain washers so they fit, or prying apart lock washers and using those.