GrayJay

Lovely time capsule of a tandem.
Note that Mafac cantilevers (and thier modern botique CX copies) do not actually have an adjustment for repositioning the pad&post height for another wheel size. Best that can be done with them would be is to angle the pad+post downward from the arm to the smaller diameter rim but then the pad will not stike the rim flat, you would need to grind the contact surface of the pads at an angle to hit the rim more squarly. I dont see a particularly compelling reason to swap from 27" to 700c for this bike. The fork and chainstay clearances are already very big, moving to slightly smaller diameter rims would only make the gaps bigger with no real benefit other than increased tire selection.

Note that wide angle cantilevers (such as Mafac) dont actually have as much mechanical advantage as later designed narrow angle cantilevers. Excellent technical dissertation on cantilever disigns here; http://www.circleacycles.com/cantile...i-geometry.pdf
If strict adherance to C&V authenticity is less of a concern than having strong brakes, switch to some medium or narrow angle cantilevers (such as from later 1980s/early 1990s MTB). Some of the early dia-comp medium angle cantilevers still look acceptably C&V and would provide much better stopping power. Some designs also have provision for up/down adjustment of the pad&post so might better accomidate 700c wheels if you did decide to switch.