Headsets with cartridge bearings on the bottom have a little more stack height, but I don't think it's enough to make a difference. Velo Orange has a nice one.

The problem I had when I replaces a 27" fork with a 700c fork is that I had to find a short reach front brake that matched the long reach brake in the rear. I don't like Weinmann/Dia Compe centerpulls, so I ended up mixing two sets of Tektro dual pivots from a bike swap. I wanted to try dual pivots anyway.