If you are deadset on going down to 700c, that will be the best choice. If you can though, dry fit a set onto your 520 and check if you have enough adjustment on your cantis first. You might luck out. If not, you can always try and find different cantis that have enough adjustment, but it will require some luck and trial and error. If anyone else here has ever made the conversion or knows of a good brakeset, please chime in here. Paul Touring brakes probably would, though it's an expensive trial and error there.

If you really like your 520 that much more, I suggest having new hubs laced to the existing rims, or even getting some new 27" rims as well, just because. The expensive route is to take it to a frame builder and get your cantilever posts rebrazed to the correct height for 700c wheels, then have your frame and fork repainted or powdercoated.

I think both bikes are terrific bikes.