Ben,
Yup, nice and simple ... and just odd enough that folks might ask what the heck the sticker is for!
Other ideas: a hood crest with "Club Cobra" replacing the "Cobra" line, the 427 side badge with "Club Cobra replacing the "Powered by Ford" (heck, replace the 427 with 302, 351, etc.); or for the greatest simplicity the standard Cobra snake emblem with "Club Cobra" on the bottom.