Originally Posted by dddd

I've done the 27-to-700c change on a 1984 720 having Shimano MC70 canti's.

What I encountered was a bit complex, and the braking leverage/power increased quite noticeably.

Rim width is a factor, as is tire width!

The rim width comes into play firstly because a narrower rim increases the pad-dive angle (which has already been increased by reducing the rim radius by those tiny 4mm).

The increased pad-dive angle reduces clearance between the edge of the brake pads and the sidewalls of the tire.
This sidewall clearance is further reduced by a wider tire jutting outward from a narrower rim. So pad geometry during setup becomes more critical (your adjustment to the pad's position/angle need to be done more precisely, to what I would call very high standards).

Due to the above considerations, I used 25mm-wide Mavic Module4 rims, even though my choice of tires was only 28mm wide.
The wider rims also moderate the increase in braking leverage caused by the lower pads, and with Aztec pads my braking is still borderline too sensitive using RSX STI levers and a shortened (from stock) straddle cable in front.

Wide rims with tall/broad brake tracks would be ideal in this case, for ease of setup and what I will call best braking.