Well a 584 with a 40mm tire is apparently pretty close in size to a 622 with a 23mm tire, or so the oft-repeat mantra around here seems to go. This is why BB drop isn't really a huge issue. That's also why I suggest checking for clearance with a 26x1 3/8 wheel with tire before going any further on the project... if it fits you should have all the room you need for fenders. A racing frame might give you adequate clearance even with smaller wheels but the fatter tires end up using all the extra space... negating the whole raison d'etre for the conversion. I haven't seen the frame so I can't really say.