There are plenty of spacers on the avid BB7's. The same ones that are on most vee brake pads. Remove those, or replace them for slightly smaller ones, so reduce it by 2.5mm, and it'll be fine on a 180mm disk.
Just remember those spacers are there to ensure you get a decent alignment between the pads and the rotor. Ensure you can still set it up straight!
Otherwise, it'll be fine. :P
Fatmike on ians account. B)