I've changed a le tour from 27's to 700c before, and I ran into problems with brake reach when I tried putting sidepulls on. What you'll find is that the new "long reach" road brakes that are available are more like the old standard reach brakes in terms of their reach. At the time I changed my wheels, I didn't have a working set of centerpulls, which would have provided plenty of reach, so I tried some funky solutions, which included an old Dia-Compe sidepull brake on the front (barely reached the rim) and a new BMX caliper brake on the rear. I never liked that setup, and when I found just what I needed on ebay to put my old centerpulls back together, I did and have been very happy with them ever since. I use kool stop pads on my centerpulls, and aero style brake levers, and the brakes are very strong. Kind of a PIA to set up, but once done, if done correctly, they stay centered. They also provide plenty of room for fenders, which I run front and rear on my bike..............Exactly how much reach you'll need will depend on your bike (there's variation even from one le tour to another depending on frame size), so in your case you might find some type of sidepull that will work. But as much as you probably hate to hear this, in going from 27's to 700c, your centerpulls are probably the best solution you're going to find. Typically, it's the rear brake that needs the most reach, btw, so you might find a brake that will reach the front (like I did with the old Dia-Compe front brake), and have to find a different solution for the rear.

One other thing you might consider is a drop bolt, which will lower the brake down from the bolt mount on the frame/fork. Harris Cyclery used to carry some, may still, but they're very expensive. You might make a homeade one (that's another thing I tried), but personally, I didn't trust it and never used it. Good luck-