Most BBK sites have a template you can download to check fitment. Need to make sure it prints out to scale though - milimeters can make all the difference in the world with clearance.
I have a Stoptech BBK, but stock 20s and had to use a spacer. I printed the template from the site, glued it to cardboard so it would be stiff and checked it against a wheel I removed. If you can find a caliper template that fits inside your 18s, then a spacer is probably all you would need. On the Stoptech template, they actually show you the OEM rotor and caliper as well as the new Stoptech rotor and calipers (obviously larger).