I'm sure the more experienced builder's will weigh in but I will make a comment or two.
I recently completed my frame building course and the mitering and placement of the cantilever posts was quite an interesting and time consuming process. I sure any competent frame builder can accomplish it for you.
About 4 years ago I did a 659b conversion , with a Miyata 610, and I was able to make the original cantilever brakes work. There is quite a bit of information available on this subject so you might be able to forego the extensive frame work.