I contemplated making this response but it's not helpful. And with adequate preparation and study, you could do it. It's a significant effort though and more expensive than it looks. I would say the rear dropouts are one of the things that new builders have the most trouble with, and adding the complication of having an existing rear triangle makes it significantly more difficult to do right. And in the end you need a paint job, which is expensive.
Seems like everything that surly makes has horizontal drops and Salsa has a lot of bikes that have adjustable dropouts. For canti, you are talking used or old stock for the most part, I think. OTOH, my LBS has some canti frames hanging from the walls. I suspect they would be willing to discount nowadays.